fix(mobile): browser composer keyboard handling on chat and draft screens

Mobile browsers don't shrink the layout for the keyboard, so the
fullscreen composer is now pinned to the visual viewport (fixed at its
offset and height, tracked as the browser pans) instead of overflowing
underneath it; the draft screen's normal composer gets the same pinning
anchored to the visible bottom via a rAF tracker, since Safari's own
focused-field reveal proved unreliable there after leaving fullscreen.
The draft and empty-session roots drop their transform-gpu (a transform
would make them the containing block for the pinned form), the app
header hides while the browser fullscreen composer is up (the form can't
out-stack it from inside the composer wrapper's stacking context), and
leaving fullscreen nudges the still-focused field back into view.
Draft starter chips now hide while the keyboard is open in browsers too,
via an oc-browser-keyboard-open root class driven by composer focus.

packages/ui/src/apps/MobileApp.tsx

@@ -1705,7 +1705,7 @@ const MobileHeader: React.FC<{
   return (
     <>
       <header
-        className="relative z-30 flex shrink-0 items-center gap-1 border-b border-border/30 bg-background/95 backdrop-blur supports-[backdrop-filter]:bg-background/80"
+        className="oc-mobile-header relative z-30 flex shrink-0 items-center gap-1 border-b border-border/30 bg-background/95 backdrop-blur supports-[backdrop-filter]:bg-background/80"
         style={{ paddingTop: 'var(--oc-safe-area-top, 0px)' }}
       >
         <div className="flex h-[var(--oc-header-height,56px)] w-full items-center gap-1 px-2">