add header threads attention menu

[tellaho]

Category: new-feature
User Impact: Users can find unread or active threads from the channel header without scanning a busy chat timeline.
Problem: Thread-level attention was too hard to parse when it lived inline with busy message rows. Users need a higher-level, repeatable shortcut for chasing unread or currently active threads across channels.
Solution: Add a ReceiptText header control beside the members action that opens one combined attention list. The badge counts unread replies only, while the dropdown merges unread and active threads and opens the selected thread directly with focus on the thread head.

File changes

desktop/src/features/channels/lib/threadAttention.ts
Adds pure derivation for the combined unread/active thread rows, badge total, row equality, preview collapsing, and coarse uptime formatting.

desktop/src/features/channels/lib/threadAttention.test.mjs
Covers row merging, sorting, unread totals, equality stabilization, preview behavior, and Ns/Nm/Nh uptime formatting.

desktop/src/features/channels/ui/ChannelThreadsAttentionMenu.tsx
Adds the header dropdown UI with the ReceiptText trigger, unread badge, combined attention rows, unread pills, and active uptime pills.

desktop/src/features/channels/ui/useThreadAttention.ts
Derives attention rows from thread unread counts, loaded timeline summaries, and thread-scoped bot typing while stabilizing row references for busy channels.

desktop/src/features/channels/ui/ChannelMembersBar.tsx
Places the new thread attention control immediately before the members/users affordance in inline headers and before the compact actions menu in compact headers.

desktop/src/features/channels/ui/ChannelScreenHeader.tsx
Passes thread attention data and selection callbacks from the channel screen into the header actions.

desktop/src/features/channels/ui/ChannelScreen.tsx
Wires the attention derivation into the channel header, includes DMs via the existing header path, and gates forums because they do not have the thread panel destination.

desktop/src/features/channels/useChannelPaneHandlers.ts
Adds a non-toggle thread-open path so selecting a dropdown item always opens and refocuses the thread instead of closing an already-open thread.

desktop/src/features/messages/useChannelTyping.ts
Scopes nested thread typing to the thread root rather than the immediate reply parent so active-thread detection aligns with thread heads.

desktop/src/features/messages/useChannelTyping.test.mjs
Adds coverage for channel-scoped typing, direct thread replies, and nested thread replies.

Reproduction Steps

1. Open a normal channel or DM that has unread thread replies and/or an active bot typing in a thread.
2. Look in the channel header immediately left of the users/members control.
3. Confirm the ReceiptText control appears and its badge reflects unread thread replies only.
4. Open the dropdown and confirm unread and active threads appear in one list, with active rows showing a coarse uptime such as 45s, 3m, or 2h.
5. Select a row and confirm the thread panel opens and scroll-focuses the selected thread head; selecting the same row again should refocus rather than close the panel.
6. Open a forum channel and confirm the control is not shown.

Screenshots/Demos

Not captured in this pass; this should get a manual click-through before merge because the navigation path was validated by code path and tests, not live app interaction.

Validation

• pnpm test -- src/features/channels/lib/threadAttention.test.mjs src/features/messages/useChannelTyping.test.mjs from desktop/ — full suite passed: 1944/1944
• pnpm typecheck from desktop/ — passed
• pnpm check from desktop/ — passed

Notes for reviewers

• Active uptime starts when the client first sees a thread as active, not from the true agent-turn start time. The true started-at signal is currently channel-scoped rather than thread-scoped.
• Threads whose heads are outside the loaded message window fall back to N replies without author/preview, but unread count and navigation still work.