feat(media): navigate attachment bundles and galleries in the viewer

[Th-Underscore]

Description

The zoom-in media viewer gains prev/next navigation on desktop:

• Attachment bundles: Zooming into an image that's part of a grouped run of consecutive media messages enables ArrowLeft/ArrowRight (and chevron buttons) to move through that run. Bundles derive from the timeline's own rendered grouping, so what you navigate matches what the timeline collapsed.
• Multi-image galleries: A single gallery message (multiple images sent as one event) navigates the same way.
• Wrap-around with edge hints: Next at the last image wraps to the first (and vice versa); the button pointing off a bundle edge rests dimmed (~20% opacity) but brightens on approach so it stays clearly clickable.
• Hover-revealed controls: On desktop the chevrons stay invisible until the cursor is over the modal (150 ms fade); arrow keys never summon them; Tab focus reveals them for keyboard users. The buttons deliberately don't inherit Sable's global button:hover lift, which previously caused an infinite hover-bounce where the hitbox chased the cursor.
• Neighbour preloading without memory spikes: A bounded sliding window warms 2 images ahead then 2 behind, recomputed as you navigate and cancelled on move/close - it only fills Sable's media caches / the HTTP cache and never retains blobs or object URLs, so even a large bundle costs at most ~2 extra images beyond what's visible. Web + encrypted bundles are skipped (their cache can't be warmed without duplicate downloads).
• Keep-stale rendering: The current image stays mounted underneath until the next resolves (spinner overlaid), eliminating blank-frame flicker; combined with preloading most swaps are instant.

Staged composer attachments: clicking a staged image preview opens the same viewer for all staged images as their own local bundle - newest-first matching the tile strip, wrap-around, edge-dimmed buttons. Save works directly from the local File (no download round-trip); removing attachments while open closes cleanly; non-image attachments don't open the viewer.

Tested on the desktop app, Ubuntu 24.04.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation - .changeset
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

I designed and directed the feature and tested it in the GUI; the code itself was AI-generated.

Bundle membership reuses the timeline collapse predicate (same sender + media type + time window) over the rendered processed rows (mediaBundle.ts pure helpers) rather than raw event arrays, as raw arrays interleave non-media rows, which broke naive adjacency. Gallery items come from the gallery content's item list in one pass. Navigation arithmetic is shared via a small wrapIndex helper in $utils/common; keyboard handling ignores events while inputs/buttons/menu items hold focus. Preloading resolves each neighbour through Sable's normal media pipeline so encryption keys register and the loopback cache warms exactly as if it had been displayed.