feat(threads): thread drawer, thread browser, live reply counts, and unread badges (#564)

📝 **Docs:** Already live at [Threads — Sable
Docs](https://docs.sable.moe/new-features/threads/) — no separate docs
PR needed.

---

### Description

Expands Matrix thread support with a full thread drawer, server-side
thread browser, live reply counts, unread badges, and several UX fixes.

#### ThreadDrawer

- Slide-in drawer for reading and replying in threads without leaving
the main room
- Thread root message shown at top with full reply history below
- Up-arrow edit-last-message: pressing ↑ in the thread input edits the
user's last own message in that thread
- Reactions add/remove trigger immediate re-renders without manual
refresh (handles the fact that reaction events carry
`m.relates_to.event_id` not `threadRootId` directly — fetches target
event to check thread membership)
- Thread root message box sized to content with `maxHeight: 200px` to
avoid dead whitespace on short messages

#### ThreadBrowser

- Lists all threads in the room; calls `room.fetchRoomThreads()` on
mount to load threads beyond the local timeline window
- Load-more button when the server has additional thread pages
- Each thread preview shows sender avatar, display name, reply count,
and last message body with ellipsis truncation
- Jump button navigates to the thread root in the main timeline (via
`useRoomNavigate`) instead of re-opening the drawer
- Thread unread badges on each preview item via
`room.getThreadUnreadNotificationCount()`

#### ThreadReplyChip (main timeline)

- Shows unread badge for threads with unseen replies via
`room.getThreadUnreadNotificationCount()`

#### Thread button (RoomViewHeader)

- If a thread drawer is open, button closes the drawer and opens the
thread browser (allows going from a single thread to the full list)
- If no thread is open, button toggles the thread browser as before
- `aria-pressed` reflects both thread browser state and open thread
drawer

Fixes #

#### Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] 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:

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

### AI disclosure:

- [x] 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)

ThreadDrawer/ThreadBrowser/thread-chip iterations were partially AI
assisted; I reviewed and adjusted logic for timeline processing, thread
loading, and event rendering behavior.

Author: 7w1

.changeset/feat-thread-enhancements.md

@@ -0,0 +1,5 @@
+---
+default: minor
+---
+
+Update threads: various fixes, browse all room threads, and see live reply counts on messages.

src/app/features/room/RoomTimeline.tsx

@@ -32,6 +32,7 @@ import { MessageBase, CompactPlaceholder, DefaultPlaceholder } from '$components
 import { RoomIntro } from '$components/room-intro';
 import { useMatrixClient } from '$hooks/useMatrixClient';
 import { useAlive } from '$hooks/useAlive';
+import { useMessageEdit } from '$hooks/useMessageEdit';
 import { useDocumentFocusChange } from '$hooks/useDocumentFocusChange';
 import { markAsRead } from '$utils/notifications';
 import {
@@ -124,6 +125,8 @@ export function RoomTimeline({
 }: Readonly<RoomTimelineProps>) {
   const mx = useMatrixClient();
   const alive = useAlive();
+
+  const { editId, handleEdit } = useMessageEdit(editor, { onReset: onEditorReset, alive });
   const { navigateRoom } = useRoomNavigate();
 
   const [hideReads] = useSetting(settingsAtom, 'hideReads');
@@ -167,7 +170,6 @@ export function RoomTimeline({
     return myPowerLevel < sendLevel;
   }, [powerLevels, mx]);
 
-  const [editId, setEditId] = useState<string>();
   const [unreadInfo, setUnreadInfo] = useState(() => getRoomUnreadInfo(room, true));
 
   const readUptoEventIdRef = useRef<string | undefined>(undefined);
@@ -472,7 +474,6 @@ export function RoomTimeline({
     room,
     mx,
     editor,
-    alive,
     nicknames,
     globalProfiles,
     spaceId: optionalSpace?.roomId,
@@ -481,8 +482,7 @@ export function RoomTimeline({
     setReplyDraft,
     openThreadId,
     setOpenThread,
-    setEditId,
-    onEditorReset,
+    handleEdit,
     handleOpenEvent: (id) => {
       const evtTimeline = getEventTimeline(room, id);
       const absoluteIndex = evtTimeline