Add experimental composer toggle for image grid/carousel layout

[mozzius]

Adds a button to toggle between grid/carousel display for 2-4 images. Has an introductory tooltip, and persists to local storage.

Simulator.Screen.Recording.-.iPhone.17.-.2026-07-06.at.17.32.54.mov

It's behind a composer:image_layout_toggle:enable flag

### Test plan

• Test posting a carousel and a grid posts
• Confirm button only shows when 2-4 images are selected

[claude]

Claude finished @mozzius's task in 3m 9s —— View job

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PR Review

• Read review guidance and CLAUDE.md
• Review the diff (origin/main...HEAD)
• Examine composer state, toggle button, embed rendering, publish + draft paths
• Post review

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Reviewed the composer state machine, the toggle button, the two viewer render paths (ImageEmbed + PostFeed), the offset helper, and the publish + draft serialization paths. The refactor from a count/gate-based layout decision to a pure embed-type decision is clean and kept consistent across both render sites (ImageEmbed.tsx:45 and PostFeed.tsx:974), and the offset helper (maybeApplyGalleryOffsetStyles.ts) now matches that same rule. One non-blocking finding.

Non-blocking: carousel choice is lost on draft save/restore

canToggleImageLayout gates the toggle to 2-4 images, and picking "carousel" moves those images into gallery media. But the layout choice for 2-4 images is encoded only in the media type (images vs gallery), and the draft round-trip discards that distinction:

• On save, both images and gallery media are serialized to the same embedGallery shape — drafts/state/api.ts:117-124.
• On restore, the variant is re-derived purely from image count — drafts/state/api.ts:532-536: restoredImages.length <= LEGACY_IMAGES_EMBED_MAX ? {type: 'images'} : {type: 'gallery'}.

Scenario: a user with the gate on composes 3 images, switches to carousel, saves the draft, then reopens it. The draft restores as type: 'images' (grid), silently reverting their choice. Since the whole feature is about letting users make and keep that choice, losing it on a draft round-trip is a real UX regression within the experiment. It's scoped to gated users composing 2-4 images and using drafts, so non-blocking — but worth deciding whether to persist the layout in the draft schema. Fix this →

Things I checked that are fine

• No lingering references to the removed PostGalleryEmbedEnable / post_gallery_embed gate anywhere in the tree.
• Existing gallery posts in the wild only have 5+ images (old imagesToMediaVariant sent <=4 to images), so "gallery always renders carousel" doesn't reclassify any existing content — all pre-existing galleries already rendered carousel.
• The reducer's embed_set_image_layout correctly re-guards with canToggleImageLayout and no-ops when the type already matches; the documented behavior that add/remove re-picks by count (dropping an explicit choice) is acceptable.
• ax in PostFeed.tsx is still used by other metrics, so no dead variable from the refactor.
• Publish path (lib/api/index.ts:350) already handles gallery media, so 2-4 image gallery drafts publish correctly.
• Strings go through Lingui, ALF/#/components used throughout, tooltip nudge mirrors the threadgate pattern, imageLayoutNudged added to the device schema.

[github-actions]

Old size	New size	Diff
8.51 MB	8.51 MB	4.17 KB (0.05%)

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