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/hooks/useDesktopRoomBridge.test.ts

@@ -0,0 +1,68 @@
+import { renderHook } from '@testing-library/react'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import * as desktopBridge from '../lib/desktopBridge'
+import { useDesktopRoomBridge } from './useDesktopRoomBridge'
+
+describe('useDesktopRoomBridge', () => {
+  beforeEach(() => {
+    vi.spyOn(desktopBridge, 'notifyRoomOpened')
+    vi.spyOn(desktopBridge, 'notifyUnreadCount')
+  })
+
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  it('calls roomOpened once when entering a room', () => {
+    renderHook(({ roomId, count }) => useDesktopRoomBridge(roomId, count), {
+      initialProps: { roomId: 'room-1', count: 0 },
+    })
+    expect(desktopBridge.notifyRoomOpened).toHaveBeenCalledTimes(1)
+  })
+
+  it('does not call roomOpened again on a rerender with the same roomId', () => {
+    const { rerender } = renderHook(({ roomId, count }) => useDesktopRoomBridge(roomId, count), {
+      initialProps: { roomId: 'room-1', count: 0 },
+    })
+    expect(desktopBridge.notifyRoomOpened).toHaveBeenCalledTimes(1)
+
+    // Unrelated rerender -- unreadRoomCount changed, roomId did not.
+    rerender({ roomId: 'room-1', count: 3 })
+    rerender({ roomId: 'room-1', count: 3 })
+    expect(desktopBridge.notifyRoomOpened).toHaveBeenCalledTimes(1)
+  })
+
+  it('calls roomOpened again on a genuine transition to a different room', () => {
+    const { rerender } = renderHook(({ roomId, count }) => useDesktopRoomBridge(roomId, count), {
+      initialProps: { roomId: 'room-1', count: 0 },
+    })
+    rerender({ roomId: 'room-2', count: 0 })
+    expect(desktopBridge.notifyRoomOpened).toHaveBeenCalledTimes(2)
+  })
+
+  it('does not call roomOpened when there is no room id', () => {
+    renderHook(({ roomId, count }) => useDesktopRoomBridge(roomId, count), {
+      initialProps: { roomId: undefined as string | undefined, count: 0 },
+    })
+    expect(desktopBridge.notifyRoomOpened).not.toHaveBeenCalled()
+  })
+
+  it('sends the unread count whenever it changes, independent of roomOpened', () => {
+    const { rerender } = renderHook(({ roomId, count }) => useDesktopRoomBridge(roomId, count), {
+      initialProps: { roomId: undefined as string | undefined, count: 0 },
+    })
+    expect(desktopBridge.notifyUnreadCount).toHaveBeenCalledWith(0)
+
+    rerender({ roomId: undefined, count: 2 })
+    expect(desktopBridge.notifyUnreadCount).toHaveBeenCalledWith(2)
+    expect(desktopBridge.notifyRoomOpened).not.toHaveBeenCalled()
+  })
+})
+
+describe('desktop bridge functions with no window.dsDesktop', () => {
+  it('remain optional -- calling them does not throw when the bridge is absent', () => {
+    expect(window.dsDesktop).toBeUndefined()
+    expect(() => desktopBridge.notifyRoomOpened()).not.toThrow()
+    expect(() => desktopBridge.notifyUnreadCount(5)).not.toThrow()
+  })
+})