Add experimental composer toggle for image grid/carousel layout

Behind the new `composer:image_layout_toggle:enable` feature gate, show
a pill button in the composer toolbar (next to the labels button) when a
post has 2-4 images. The button switches the draft between the legacy
`app.bsky.embed.images` shape (grid) and the newer
`app.bsky.embed.gallery` shape (carousel, previously only used for 5+
images). Gallery embeds now always render with the carousel layout so
the choice is reflected for viewers.

The button is wrapped in a one-time tooltip nudge, following the same
pattern as the threadgate button, and emits a
`composer:imageLayout:toggle` metric.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SvDXhHEBCjAfokEFK8pHt8

src/storage/hooks/image-layout-nudged.ts

@@ -0,0 +1,9 @@
+import {device, useStorage} from '#/storage'
+
+export function useImageLayoutNudged() {
+  const [imageLayoutNudged = false, setImageLayoutNudged] = useStorage(device, [
+    'imageLayoutNudged',
+  ])
+
+  return [imageLayoutNudged, setImageLayoutNudged] as const
+}

src/storage/schema.ts

@@ -62,6 +62,7 @@ export type Device = {
   demoMode: boolean
   activitySubscriptionsNudged?: boolean
   threadgateNudged?: boolean
+  imageLayoutNudged?: boolean
   inviteFriendsFollowersPromoDismissed?: boolean
   /**
    * Selected color theme for the Invite Friends QR card.