feat(call): full-screen call when a phone is turned sideways (#126)

Landscape, at most 500px tall, touch input (a sideways phone is ~840px
wide, the Tablet size class, so the trigger is the screen shape), in this
room's joined call, chat closed: the call takes the whole screen. No room
header, sidebar or call-status bar; safe-area insets respected.

- Taps inside the call frame never reach the app, so while the controls
  are hidden a transparent layer covers the call: the first tap shows the
  call bar plus Chat / Exit full screen, which hide again after 4 s
  without a touch; with them up the call underneath stays usable.
- In an Android browser tab the first tap also enters the browser's own
  full screen (hides the address bar); left again on rotate back, hang
  up or exit. No effect on iPhone or the installed app.
- Exit full screen lasts until the phone is upright again.
- The frame is transparent (room wallpaper), so in full screen it gets an
  opaque base colour under the wallpaper.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/components/CallEmbedProvider.tsx

@@ -37,7 +37,7 @@ import {
   useCallThemeSync,
   useCallStart,
 } from '../hooks/useCallEmbed';
-import { callChatAtom, callEmbedAtom } from '../state/callEmbed';
+import { callChatAtom, callEmbedAtom, immersiveCallAtom } from '../state/callEmbed';
 import { toastQueueAtom } from '../state/toast';
 import { CallEmbed, useCallControlState, useScreensharePromptOpen } from '../plugins/call';
 import { hangupCallAndWait } from '../plugins/call/hangup';
@@ -954,6 +954,7 @@ export function CallEmbedProvider({ children }: CallEmbedProviderProps) {
   const inCallRoom = callEmbed && selectedRoom === callEmbed.roomId;
   const callActive = callEmbed && joined;
   const callVisible = inCallRoom && callActive && !chatOnlyView;
+  const immersiveCall = useAtomValue(immersiveCallAtom);
   const pipMode = callActive && !inCallRoom;
   const pipPromptOpen = useScreensharePromptOpen(callEmbed?.control);
   const { navigateRoom } = useRoomNavigate();
@@ -1458,7 +1459,13 @@ export function CallEmbedProvider({ children }: CallEmbedProviderProps) {
           ...(pipMode
             ? { background: color.Surface.Container }
             : callVisible
-              ? wallpaperStyle
+              ? {
+                  // [Gitea #126] Full screen: the frame is transparent, so give
+                  // it an opaque base under the wallpaper or the room's
+                  // sidebar / call bar show through.
+                  ...(immersiveCall ? { backgroundColor: color.Background.Container } : {}),
+                  ...wallpaperStyle,
+                }
               : {}),
         }}
         ref={callEmbedRef}