fix(dashboard): portal MobileNavBar to document.body for robust fixed positioning

vpdebug on Android tablet captured the nav bar's getBoundingClientRect
at y=2800 with html height 798 — meaning some ancestor was creating a
containing block for the bar's `position: fixed`, putting it 2000+ px
below the visible window. Any ancestor with `transform`, `filter`,
`will-change`, or `contain: paint` would do that, and it's hard to
audit which of our many ancestors (providers, view wrappers, plugin
hosts) introduces one in a given Android Chrome state.

Sidestep the whole question: render the bar via createPortal into
document.body. Its only ancestor is body, so `position: fixed; bottom: 0`
is now guaranteed to pin to the visual viewport bottom.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/MobileNavBar.tsx

@@ -1,5 +1,6 @@
 import "./MobileNavBar.css";
 import { useCallback, useEffect, useLayoutEffect, useMemo, useRef, useState, type ReactNode } from "react";
+import { createPortal } from "react-dom";
 import {
   Activity,
   Bot,
@@ -296,7 +297,12 @@ export function MobileNavBar({
     || view === "stash-recovery"
     || (isPluginViewId(view) && !topLevelPrimaryPluginViews.some((entry) => buildPluginTaskViewId(entry.pluginId, entry.view.viewId) === view));
 
-  return (
+  // Portal the bar directly into document.body so its `position: fixed`
+  // can't be hijacked by an ancestor's containing block (any ancestor with
+  // transform/filter/will-change/contain creates a fixed-positioning
+  // containing block — and we hit exactly that on Android tablet, leaving
+  // the bar positioned far below the visible window).
+  const content = (
     <>
       <nav
         ref={navRef}
@@ -837,4 +843,6 @@ export function MobileNavBar({
       )}
     </>
   );
+  if (typeof document === "undefined") return content;
+  return createPortal(content, document.body);
 }