refactor: drop mobile keyboard/viewport hacks, rely on browser

Remove body position:fixed lock, visualViewport listener in MainLayout
(keyboard-inset heuristics, scroll-to-zero lock, focusin/focusout RAF),
[data-keyboard-avoid-active] translateY rule, and related CSS vars
(--oc-keyboard-inset, --oc-keyboard-avoid-offset, --oc-keyboard-home-indicator,
--oc-visual-viewport-offset-top). Strip data-keyboard-avoid* attrs and
keyboardAvoid props from Dialog/ScrollableOverlay and consumers. Drop
isKeyboardOpen from useUIStore.

With body unlocked and the layout plain flex-col (h-dvh), the browser
shrinks the viewport on keyboard open naturally — composer sits above
the keyboard, header stops lagging, input no longer jitters.

FilesView.nudgeEditorSelectionAboveKeyboard now derives the occluded
bottom locally from visualViewport + documentElement.clientHeight.

packages/ui/src/components/sections/openchamber/OpenChamberPage.tsx

@@ -32,7 +32,6 @@ export const OpenChamberPage: React.FC<OpenChamberPageProps> = ({ section }) =>
     if (!section) {
         return (
             <ScrollableOverlay
-                keyboardAvoid
                 outerClassName="h-full"
                 className="w-full"
             >
@@ -95,7 +94,6 @@ export const OpenChamberPage: React.FC<OpenChamberPageProps> = ({ section }) =>
 
     return (
         <ScrollableOverlay
-            keyboardAvoid
             outerClassName="h-full"
             className="w-full"
         >