[Layout] Base (#6907)

* Add common gutter styles as hook

* Add computed scrollbar gutter CSS vars

* Add new layout components

* Replace layout components in settings screens

* Remove old back button

* Invert web border logic for easier migration

* Clean up Slot API

* Port over FF handling of scrollbar offset

* Trade boilerplate for ease of use

* Limit to one line

* Allow two lines, fix wrapping

* Fix alignment

* sticky headers

* set max with on header and center

* [Layout] Notifications Header (#6910)

* Replace notifications screen header

* fix cropped indicator

---------

Co-authored-by: Samuel Newman <mozzius@protonmail.com>

* Replace Hashtag header (#6928)

* [Layout] ChatList header (#6929)

* Replace ChatList header

* update chat settings as well

---------

Co-authored-by: Samuel Newman <mozzius@protonmail.com>

* Add web borders to Chat settings

* Remove unused var

* Move ChatList header outside center

* Replace empty chat layout

* fix breakpoints

* [Layout] Scrollbar gutters (#6908)

* Fix sidebar alignment

* Make sure scrollbars don't hide

* Gift left nav more space

* Use stable one-edge, update logic in RightNav

* Ope

* Increase width

* Reset

* Add transform to sidebars

* Remove bg in sidebars

* Handle shifts in layout components

* Replace scroll-removal handling

* Make react-remove-scroll an explicit dep

* Remove unused script

* use correct scroll insets (#6950)

* [Layout] Feeds headers (#6913)

* Replace ViewHeader internals, duplicate old ViewHeader

* Replace Feeds header

* Replace SavedFeeds header

* Visual alignment

* Uglier but clear

* Use old ViewHeader for SavedFeeds

* use Layout.Center instead of Layout.Content

* use left-aligned header for feed edit

* delete unused old view header

---------

Co-authored-by: Samuel Newman <mozzius@protonmail.com>

* [Layout] Every other screen (#6953)

* attempt to fix double borders on every other screen

* delete ListHeaderDesktop

* delete `SimpleViewHeader` and fix screens (#6956)

* Make Layout.Center not full height

* Refactor List to use Layout.Center, remove built-in borders

* Fix Home screen

* Refactor PagerWithHeader to use Layout components

* Replace components in ProfileFeed and ProfileList

* Borders on Profile

* Search screen replacements

* use new header for profile subpage header (#6958)

* Search AutocompleteResults

* use new header for starter pack wizard (#6957)

* Fix post thread

* Enable borders by default

* Moderation muted and blocked accounts

* Fix scrollbar offset on Labeler

* Remove ScrollView from Moderation

* Remove ScrollView from Deactivated

* Remove ScrollView from onboarding

* Remove ScrollView from SignupQueued

* Mark deprecations

* fix lint

* Fix double borders on profile load

* Remove unneeded CenteredView from noty Feed

* Remove double Center layout on Notifications screen

* Remove double Center layout on ChatList screen

* Handle scrollbar offset in chat

* Use new atom for other scrollbar offsets

* Remove borders from old views

* Better doc

* Remove temp migration prop

* Fix new atom usage on native

* Clean up Hashtag screen

* Layout docs

* Clarify usage in Pager

* Handle nested offset contexts

* Clean up Layout

* fix feeds page

* asymmetric header on native (#6969)

* Reusable header const

* Fix up home header

* Add back button to convo

* Add hitslop to header buttons

* Comment

* Better handling on native for new atom

* Format

* Fix nested flatlist on mod screens

* Use react-remove-scroll-bar directly

* Fix notification count overflow on web

* Clarify doc

---------

Co-authored-by: Samuel Newman <mozzius@protonmail.com>

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+# Layout
+
+This directory contains our core layout components. Use these when creating new
+screens, or when supplementing other components with functionality like
+centering.
+
+## Usage
+
+If we aren't talking about the `shell` components, layouts on individual screens
+look like more or less like this:
+
+```tsx
+<Outer>
+  <Header>...</Header>
+  <Content>...</Content>
+</Outer>
+```
+
+I'll map these words to real components.
+
+### `Layout.Screen`
+
+Provides the "Outer" functionality for a screen, like taking up the full height
+of the screen. **All screens should be wrapped with this component,** probably
+as the outermost component.
+
+> [!NOTE]
+> On web, `Layout.Screen` also provides the side borders on our central content
+> column. These borders are fixed position, 1px outside our center column width
+> of 600px.
+>
+> What this effectively means is that _nothing inside the center content column
+> needs (or should) define left/right borders._ That is now handled in one
+> place: within `Layout.Screen`.
+
+### `Layout.Header.*`
+
+The `Layout.Header` component actually contains multiple sub-components. Use
+this to compose different versions of the header. The most basic version looks
+like this:
+
+```tsx
+<Layout.Header.Outer>
+  <Layout.Header.BackButton /> {/* or <Layout.Header.MenuButton /> */}
+
+  <Layout.Header.Content>
+    <Layout.Header.TitleText>Account</Layout.Header.TitleText>
+
+    {/* Optional subtitle */}
+    <Layout.Header.SubtitleText>Settings for @esb.lol</Layout.Header.SubtitleText>
+  </Layout.Header.Content>
+
+  <Layout.Header.Slot />
+</Layout.Header.Outer>
+```
+
+Note the additional `Slot` component. This is here to keep the header balanced
+and provide correct spacing on all platforms. The `Slot` is 34px wide, which
+matches the `BackButton` and `MenuButton`.
+
+> If anyone has better ideas, I'm all ears, but this was simple and the small
+> amount of boilerplate is only incurred when creating a new screen, which is
+> infrequent.
+
+It can also function as a "slot" for a button positioned on the right side. See
+the `Hashtag` screen for an example, abbreviated below:
+
+```tsx
+<Layout.Header.Slot>
+  <Button size='small' shape='round'>...</Button>
+</Layout.Header.Slot>
+```
+
+If you need additional customization, simply use the components that are helpful
+and create new ones as needed. A good example is the `SavedFeeds` screen, which
+looks roughly like this:
+
+```tsx
+<Layout.Header.Outer>
+  <Layout.Header.BackButton />
+
+  {/* Override to align content to the left, making room for the button */}
+  <Layout.Header.Content align='left'>
+    <Layout.Header.TitleText>Edit My Feeds</Layout.Header.TitleText>
+  </Layout.Header.Content>
+
+  {/* Custom button, wider than 34px */}
+  <Button size='small'>...</Button>
+</Layout.Header.Outer>
+```
+
+> [!TIP]
+> The `Header` should be _outside_ the `Content` component in order to be
+> fixed on scroll on native. Placing it inside will make it scroll with the rest
+> of the page.
+
+### `Layout.Content`
+
+This provides the "Content" functionality for a screen. This component is
+actually an `Animated.ScrollView`, and accepts props for that component. It
+provides a little default styling as well. On web, it also _centers the content
+inside our center content column of 600px_.
+
+> [!NOTE]
+> What about flatlists or pagers? Those components are not colocated here (yet).
+> But those components serve the same purpose of "Content".
+
+## Examples
+
+The most basic layout available to us looks like this:
+
+```tsx
+<Layout.Screen>
+  <Layout.Header.Outer>
+    <Layout.Header.BackButton /> {/* or <Layout.Header.MenuButton /> */}
+
+    <Layout.Header.Content>
+      <Layout.Header.TitleText>Account</Layout.Header.TitleText>
+
+      {/* Optional subtitle */}
+      <Layout.Header.SubtitleText>Settings for @esb.lol</Layout.Header.SubtitleText>
+    </Layout.Header.Content>
+
+    <Layout.Header.Slot />
+  </Layout.Header.Outer>
+
+  <Layout.Content>
+    ...
+  </Layout.Content>
+</Layout.Screen>
+```
+
+**For `List` views,** you'd sub in `List` for `Layout.Content` and it will
+function the same. See `Feeds` screen for an example.
+
+**For `Pager` views,** including `PagerWithHeader`, do the same. See `Hashtag`
+screen for an example.
+
+## Utilities
+
+### `Layout.Center`
+
+This component behaves like our old `CenteredView` component.
+
+### `Layout.SCROLLBAR_OFFSET` and `Layout.SCROLLBAR_OFFSET_POSITIVE`
+
+Provide a pre-configured CSS vars for use when aligning fixed position elements.
+More on this below.
+
+## Scrollbar gutter handling
+
+Operating systems allow users to configure if their browser _always_ shows
+scrollbars not. Some OSs also don't allow configuration.
+
+The presence of scrollbars affects layout, particularly fixed position elements.
+Browsers support `scrollbar-gutter`, but each behaves differently. Our approach
+is to use the default `scrollbar-gutter: auto`. Basically, we start from a clean
+slate.
+
+This handling becomes particularly thorny when we need to lock scroll, like when
+opening a dialog or dropdown. Radix uses the library `react-remove-scroll`
+internally, which in turn depends on
+[`react-remove-scroll-bar`](https://github.com/theKashey/react-remove-scroll-bar).
+We've opted to rely on this transient dependency. This library adds some utility
+classes and CSS vars to the page when scroll is locked.
+
+**It is this CSS variable that we use in `SCROLLBAR_OFFSET` values.** This
+ensures that elements do not shift relative to the screen when opening a
+dropdown or dialog.
+
+These styles are applied where needed and we should have very little need of
+adjusting them often.