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/styles/mobile.css

@@ -165,16 +165,6 @@
     overflow: hidden;
   }
 
-  /* Prevent iOS keyboard from scrolling the page */
-  :root.device-mobile:not(.desktop-runtime) body,
-  :root.device-mobile:not(.desktop-runtime) #root {
-    position: fixed;
-    inset: 0;
-    width: 100%;
-    overflow: hidden;
-    overscroll-behavior-y: none;
-  }
-
   /* Fix main layout container */
   :root.mobile-pointer:not(.desktop-runtime) .flex.flex-col.h-screen {
     height: 100vh;
@@ -182,16 +172,6 @@
     min-height: 0;
   }
 
-  /* Fix header positioning */
-  :root.device-mobile:not(.desktop-runtime) .header-safe-area {
-    position: relative;
-    top: auto;
-    left: auto;
-    right: auto;
-    width: 100%;
-    z-index: 10;
-  }
-
   /* Fix main content area */
   :root.mobile-pointer:not(.desktop-runtime) .flex-1.overflow-hidden {
     min-height: 0;
@@ -287,22 +267,6 @@
       padding-bottom: var(--oc-safe-area-bottom-visual);
     }
 
-    /* iOS keyboard home indicator safe area - used when keyboard is open */
-    :root.device-mobile:not(.desktop-runtime) .ios-keyboard-safe-area {
-      padding-bottom: calc(var(--oc-keyboard-home-indicator, 34px) + var(--oc-safe-area-bottom-visual, 0px)) !important;
-    }
-
-    /* Prevent keyboard-induced page scroll on iOS PWA */
-    :root.device-mobile:not(.desktop-runtime) body {
-      min-height: 100vh;
-      min-height: -webkit-fill-available;
-      position: fixed;
-      inset: 0;
-      width: 100%;
-      overflow: hidden;
-      overscroll-behavior-y: none;
-    }
-
     /* Fix iOS viewport issues */
     :root.device-mobile:not(.desktop-runtime) .flex.flex-col.h-screen {
       min-height: 100vh;