[web] mobile action sheet: wire reactions-pins emoji recency + full picker route

[intendednull]

Tracking issue for the reactions-pins.md TODOs in the mobile action sheet at crates/web/src/components/message.rs:1343 and :1406. Spec at docs/specs/2026-04-19-ui-design/reactions-pins.md.

Distinct from #186 (desktop hover-toolbar "more reactions" button). The two sites here are inside the long-press action sheet rendered for touch viewports.

TODO sites

• crates/web/src/components/message.rs:1343 — quick-emoji row hardcodes REACTION_EMOJI.iter().take(6). Spec wants channel-scoped recency.
• crates/web/src/components/message.rs:1406 — "add reaction" sheet item on:click is a no-op ev.stop_propagation(). Should route to the full emoji picker.

Scope

1. Channel-scoped emoji recency list (lives where? — likely ChatMeta or ProfileState; confirm during plan). Replace the REACTION_EMOJI slice in the sheet.
2. Reuse the emoji picker component from Hover-toolbar "more reactions" button opens full action dropdown instead of emoji picker #186 once it lands. Wire the "add reaction" sheet item to open the picker anchored to / overlaying the sheet (decide UX during plan — replace sheet vs. stack over).
3. Browser test asserting recency-driven order in the sheet's quick-emoji row + that "add reaction" opens the picker (not the no-op).

Out of scope

• Desktop hover-toolbar emoji picker (Hover-toolbar "more reactions" button opens full action dropdown instead of emoji picker #186).
• Pinned-message panel redesign (separate reactions-pins.md section).
• Reactor-list popover.

Refs

• Spec: docs/specs/2026-04-19-ui-design/reactions-pins.md
• Sibling issue (desktop): Hover-toolbar "more reactions" button opens full action dropdown instead of emoji picker #186
• Filed via audit cleanup (audit F34 [debt]: 10 stale TODO(spec.md) markers reference unresolved external specs #534)