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/hooks/useChatAutoFollow.ts

@@ -752,8 +752,13 @@ export const useChatAutoFollow = ({
                 // Only pinned content rides the keyboard; a user reading history
                 // stays put (the composer slides over the bottom, like native apps).
                 if (stateRef.current !== 'following' || !canScroll(el)) return;
+                // Fold any pending re-pin distance into the same slide. The pill
+                // composer swaps to the full composer right before focusing, which
+                // shrinks the viewport without moving scrollTop — compensating it
+                // here makes keyboard + composer growth one motion instead of two.
+                const pending = Math.max(0, el.scrollHeight - el.scrollTop - el.clientHeight);
                 inner.style.transition = transition;
-                inner.style.transform = `translateY(${-detail.slide}px)`;
+                inner.style.transform = `translateY(${-(detail.slide + pending)}px)`;
                 return;
             }