feat: add centralized permission notification system for agent/subagent requests

Implements a unified permission notification UI that adapts to different runtime
environments (Electron desktop vs web browser) with distinct visual presentations.

## What Changed

### New Components
- `permission-notification-banner.tsx`: Adaptive notification component
  * Electron (desktop): Full banner with "⚠️ Approval Required" text and count badge
  * Web browser (portrait): Circular indicator badge showing pending count
- `permission-approval-modal.tsx`: Interactive modal for reviewing/approving permissions
  * Displays permission type, detailed message, and diff viewer for file changes
  * Keyboard shortcuts: Enter (allow once), A (always), D (deny), Esc (close)
  * Queue management with "X of Y" counter for multiple pending permissions
- `permission-notification.css`: Comprehensive styling with pulsing animations

### Integration
- Updated `instance-shell2.tsx`:
  * Added banner to desktop center toolbar (next to Command Palette)
  * Added banner to mobile/phone layout center section
  * Added modal component for permission approval workflow
- Updated `controls.css`: Imported new permission notification styles

## Why This Change

**Before**: Permission requests had no visual indicator in the UI, making it
difficult for users to know when agent/subagent actions required approval.

**After**: Users receive clear, persistent visual notifications with:
- Pulsing animation to draw attention
- Environment-appropriate UI (full banner on desktop, compact badge on web)
- Click-to-review workflow with full permission details

## Benefits

1. **Better UX**: Users immediately see when permissions need approval
2. **Responsive Design**: Adapts to desktop (Electron) and web browser contexts
3. **Accessible**: Proper ARIA labels, keyboard shortcuts, and focus management
4. **Queue Management**: Handles multiple pending permissions gracefully
5. **Contextual Information**: Shows diffs for file changes, permission types, etc.

## Impact

- **No Breaking Changes**: Purely additive feature
- **Build**: ✅ Verified successful build
- **Testing**: ✅ Tested in Electron app and web browser

packages/ui/src/components/permission-notification-banner.tsx

@@ -0,0 +1,57 @@
+import { Show, createMemo, type Component } from "solid-js"
+import { getPermissionQueueLength } from "../stores/instances"
+import { isElectronHost } from "../lib/runtime-env"
+
+interface PermissionNotificationBannerProps {
+  instanceId: string
+  onClick: () => void
+}
+
+const PermissionNotificationBanner: Component<PermissionNotificationBannerProps> = (props) => {
+  const queueLength = createMemo(() => getPermissionQueueLength(props.instanceId))
+  const hasPermissions = createMemo(() => queueLength() > 0)
+  const isElectron = isElectronHost()
+
+  return (
+    <Show when={hasPermissions()}>
+      {/* Electron: Full banner with text */}
+      <Show when={isElectron}>
+        <button
+          type="button"
+          class="permission-notification-banner"
+          onClick={props.onClick}
+          aria-label={`${queueLength()} permission${queueLength() > 1 ? "s" : ""} pending approval`}
+        >
+          <span class="permission-notification-icon" aria-hidden="true">
+            ⚠️
+          </span>
+          <span class="permission-notification-text">
+            Approval Required
+          </span>
+          <Show when={queueLength() > 1}>
+            <span class="permission-notification-count" aria-label={`${queueLength()} permissions`}>
+              {queueLength()}
+            </span>
+          </Show>
+        </button>
+      </Show>
+
+      {/* Web: Compact indicator button */}
+      <Show when={!isElectron}>
+        <button
+          type="button"
+          class="permission-indicator-button"
+          onClick={props.onClick}
+          aria-label={`${queueLength()} permission${queueLength() > 1 ? "s" : ""} pending approval. Click to review.`}
+          title={`${queueLength()} permission${queueLength() > 1 ? "s" : ""} pending approval`}
+        >
+          <span class="permission-indicator-badge">
+            {queueLength() > 9 ? "9+" : queueLength()}
+          </span>
+        </button>
+      </Show>
+    </Show>
+  )
+}
+
+export default PermissionNotificationBanner