feat(a11y): screen-reader announcements for call events (#168)

A visually hidden aria-live=polite region (rendered by CallEmbedProvider so it
outlives the embed) announces joins/leaves — batched over 1.5 s: 'alice
joined', 'alice and bob joined', '3 people joined' — your own mute/unmute,
deafen/undeafen and screenshare start/stop, and 'Call ended'. Nothing visible,
nothing audible for anyone else, no setting. Verified headless by observing
the region: bob joined → You are muted → … → bob left → Call ended.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/state/callAnnouncement.ts

@@ -0,0 +1,8 @@
+import { atom } from 'jotai';
+
+/**
+ * [Gitea #168] Latest screen-reader-only call announcement ("alice joined",
+ * "You are muted", "Call ended"). Rendered by CallEmbedProvider in an
+ * aria-live region that outlives the call embed, so "Call ended" is heard.
+ */
+export const callAnnouncementAtom = atom<string>('');