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/lib/desktopBridge.ts

@@ -15,6 +15,12 @@ declare global {
   interface Window {
     dsDesktop?: {
       setUnreadCount?(unreadRoomCount: number): void
+      // Tells the tray/dock icon the user has genuinely entered a room --
+      // the desktop wrapper's own signal to clear that room's tray
+      // indicator. Deliberately not fired just because the window was
+      // shown/focused/restored, or opened from the tray: none of those mean
+      // the user actually looked at the room the unread message is in.
+      roomOpened?(): void
       showNotification?(notification: DesktopNotificationRequest): void
       onNotificationClick?(callback: (roomId: string) => void): () => void
     }
@@ -38,6 +44,16 @@ export function isDesktopClickListenerSupported(): boolean {
   return typeof window.dsDesktop?.onNotificationClick === 'function'
 }
 
+// Both no-op silently if the bridge or this specific method isn't present,
+// same as showDesktopNotification below -- callers never need to guard.
+export function notifyRoomOpened(): void {
+  window.dsDesktop?.roomOpened?.()
+}
+
+export function notifyUnreadCount(unreadRoomCount: number): void {
+  window.dsDesktop?.setUnreadCount?.(unreadRoomCount)
+}
+
 // No-ops silently if the bridge or this specific method isn't present --
 // callers don't need to guard, matching the rest of this module's
 // capability-detect-per-method philosophy (see #49's "feature-test each