Fix desktop tray unread indicator: clear only on genuine room entry

The tray indicator must stay visible while a message is unread and
clear only when the user actually opens the room containing it --
never merely because the window was shown, focused, restored, or
opened from the tray.

Adds window.dsDesktop.roomOpened() (called exactly once per genuine
transition to a real room id, via useDesktopRoomBridge) and wires up
window.dsDesktop.setUnreadCount() (declared but never actually called
before this) to fire whenever the unread-room count changes. Both are
no-ops in browser builds, matching this bridge's existing optional-
chaining convention.

Frontend unread state stays authoritative -- clearRoomIndicators is
pulled out of ChatShellPage as a pure function (same behavior, now
directly testable) so opening one room only clears that room's
unread/mention flags, leaving every other room's state untouched.

Also adds a frontend test runner (vitest + React Testing Library) --
none existed in this project before. 10 new tests cover: roomOpened
firing once on entry, not refiring on an unrelated rerender with the
same room id, refiring on a genuine room-to-room transition, never
firing with no room id, setUnreadCount firing independently of
roomOpened, clearing one room preserving every other room's unread
state, and the desktop bridge remaining fully optional.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/ChatShellPage.tsx

@@ -1,4 +1,4 @@
-import { useCallback, useEffect, useState } from 'react'
+import { useCallback, useEffect, useMemo, useState } from 'react'
 import { useNavigate, useParams } from 'react-router-dom'
 import { NetworkError } from '../api/client'
 import { listMyRooms, listRoomMembers } from '../api/rooms'
@@ -12,7 +12,9 @@ import { Sidebar } from '../components/Sidebar'
 import { TopBar } from '../components/TopBar'
 import { useAuth } from '../context/AuthContext'
 import { useChatSocketContext } from '../context/ChatSocketContext'
+import { useDesktopRoomBridge } from '../hooks/useDesktopRoomBridge'
 import { MOBILE_BREAKPOINT, useWindowWidth } from '../hooks/useWindowWidth'
+import { clearRoomIndicators } from '../lib/roomIndicators'
 import type { MyRoomItem, RoomMember } from '../types'
 import './ChatShellPage.css'
 
@@ -60,10 +62,13 @@ export function ChatShellPage() {
 
   const socket = useChatSocketContext()
 
-  const clearRoomIndicators = useCallback((id: string) => {
-    setRooms((prev) => prev.map((r) => (r.id === id ? { ...r, has_unread: false, has_mention: false } : r)))
+  const handleRoomRead = useCallback((id: string) => {
+    setRooms((prev) => clearRoomIndicators(prev, id))
   }, [])
 
+  const unreadRoomCount = useMemo(() => rooms.filter((r) => r.has_unread).length, [rooms])
+  useDesktopRoomBridge(activeRoom?.id, unreadRoomCount)
+
   useEffect(
     () =>
       socket.subscribe((envelope) => {
@@ -134,7 +139,7 @@ export function ChatShellPage() {
               onToggleInfo={() => setInfoOpen((v) => !v)}
               infoOpen={infoOpen}
               socket={socket}
-              onRoomRead={clearRoomIndicators}
+              onRoomRead={handleRoomRead}
             />
           ) : (
             !isMobile && (