feat: add grid layout for Material/Samsung collection attachments (5/7)

[DeveloperBlue]

Adds a Google Message/Material-inspired grid for multi-attachment media collections on Material and Samsung skins.

Built on top of #3176 (collage layout) and continues toward #3147 / #3122.

• New CollectionGroupGrid — composable layout bands on a shared 3-column grid
• Route Material / Samsung collections (2+) to grid; iOS collage/stack routing unchanged
• +N overflow cell when count > 7 — tap opens fullscreen; long-press expands remaining items inline
• Author-edge reaction overhang + tightened corners when subject/body is adjacent
• bugfix: drop extra avatar inset on collections
• bugfix: restore bottom spacing under Material reply bubble

I had a lot of fun with this one, the way the grid composes itself and expands is pretty nice.

2–3 Items	+N Inline Expand
	long-press-expand.mp4
7 Items + Reactions	5 Items + Subject/Body

Changes (commit-by-commit)

1. CollectionGroupGrid — Material/Samsung grid widget (bands, chrome, reactions, download wrap)
2. Route Material/Samsung → grid — MessagePartContent / layout resolve uses grid for non-iOS collections
3. Reaction clipping — keep grid cell reactions from clipping; tighter author-edge overhang
4. No card shadows on tiles — flat grid chrome vs collage/stack cards
5. +N see-more cell — overlay count when truncated; tap → fullscreen
6. Square media in cells — cover-fill without letterboxing inside grid tiles
7. Outer silhouette clip — clip the whole grid instead of per-cell rounding
8. Author-edge corner tighten — when subject or body sits above/below the grid
9. Docs — grid layout + routing notes in collections/CLAUDE.md
10. Material reply spacing — missing bottom gap under reply bubble
11. Drop extra avatar inset on collections
12. Composable bands rework — banner / HeroStack(n) / SquareRow(n) packing for expanded layouts
13. Skin corner radii — Material / Samsung / iOS radius tuning
14. Long-press +N inline expand — load remaining items in place + haptics

Per-file changes

collections/collection_group_grid.dart

New grid layout: composable bands, +N truncate/expand, silhouette clip, reactions, download wrap.

collections/collection_attachment_card.dart

Grid-aware chrome (no shadow, square cells, reaction overhang).

message_part_content.dart

Route Material/Samsung media collections to CollectionGroupGrid.

resolved_file_content.dart

Square/cover fill behavior for grid cells.

message_holder.dart / reply_bubble_section.dart

Avatar inset + Material reply bottom spacing fixes.

collections/CLAUDE.md

Routing table (iOS collage/stack vs Material/Samsung grid) and grid ownership notes.

Partially addresses #3122 and #2605 and lays groundwork for #2812