fix(mobile): voice join flow — correct screen key + remove desktop PiP from mobile branch

Two compounding root causes produced a "PiP-style grey view" briefly visible after Join Voice on mobile:

1. MobileSpacesScreen.handleVoiceJoin pushed 'voice' onto the mobile stack. MobileShell.screenMap only has 'voice-full' — the renderer returned null for 'voice' while the root screen sat under `visibility: hidden` (stack length > 0). Corrected to 'voice-full' with an explanatory comment.
2. AppLayout mounted the desktop <PictureInPicture /> in the mobile branch too. Its visibility check `(isInServerVoice || isInDmCall) && !voiceFullscreen` is true the moment currentVoiceChannelId is set, so the desktop PiP rendered as the only visible UI on top of the hidden mobile shell. Removed from the mobile branch; kept on desktop. Added a comment to prevent regression.

Mobile owns its voice overlay chrome exclusively via MobileVoiceMiniBar + MobileVoiceFullScreen.

Spec: docs/systems/mobile-ui.md gains a "Voice Join Flow (Mobile)" subsection and a leading paragraph in "Voice Overlay" stating that PiP is desktop-only.

packages/web/src/components/layout/AppLayout.tsx

@@ -406,7 +406,13 @@ export function AppLayout() {
         <UserProfileModal />
         <IncomingCallModal />
         <ImagePreview />
-        <PictureInPicture />
+        {/* PictureInPicture is desktop-only. Mobile has its own purpose-built
+            voice chrome (MobileVoiceMiniBar + MobileVoiceFullScreen) mounted
+            inside MobileShell. Mounting PiP here too caused a "PiP-style grey
+            view" to appear on top of the mobile shell whenever a voice call
+            was connected but the voice-full screen was not on the stack
+            (e.g. immediately after Join, or after popping voice-full back to
+            the root). */}
         <SoundController />
         <GlobalAudioRenderer />
         <NotificationController />