feat(mobile): collapsed pill composer and mobile input redesign

Mobile composer redesign: when the keyboard is closed the input collapses
into a narrow pill (sessions, attach, placeholder, mic) with a round
new-session button that fades away on the draft screen. Model and agent
selectors move into a row above the textarea; the draft project/branch
pickers and the attachment menu become searchable bottom sheets reusing
MobileOverlayPanel; a drag handle (also available while dictating) swipes
the composer into and out of a fullscreen mode.

Keyboard-lifecycle hardening: composer controls (agent cycle, dictation
and its overlay controls) no longer steal focus and dismiss the keyboard;
overlays reopen the keyboard on close via a debounced restore chain that
survives menu-to-picker handoffs and skips the native file picker; open
overlays and dictation keep the composer expanded. Dictation starts
directly from the pill and its overlay content fades in after the shape
settles. The keyboard slide compensates the pill-to-full height change in
one motion, and the mobile highlight mirror is disabled so the caret
always matches the text layout.

packages/ui/src/components/chat/ChatContainer.tsx

@@ -535,7 +535,9 @@ export const ChatContainer: React.FC<ChatContainerProps> = ({ autoOpenDraft = tr
     const chatSurfaceMode = useChatSurfaceMode();
     const draftOpen = Boolean(newSessionDraft?.open);
     const initError = useGlobalSyncStore((s) => s.error);
-    const isDesktopExpandedInput = isExpandedInput && !isMobile;
+    // Despite the historical name, this now covers mobile too: the mobile
+    // composer enters the same fullscreen-input mode via its drag handle.
+    const isDesktopExpandedInput = isExpandedInput;
     const useCompactDraftLayout = isMobile || isVSCode || chatSurfaceMode === 'mini-chat';
     const messageListRef = React.useRef<MessageListHandle | null>(null);
     const draftProjectLabel = React.useMemo(() => {