fix: toolbar pushed off-screen on iPad Safari with tabs

On iPad Safari with the tab bar visible, `100vh` extends behind the
browser chrome, pushing the fixed-position toolbar out of view.

- Add `viewport-fit=cover` to viewport meta tag
- Use `100dvh` with `100vh` fallback for body/.app height
- Set `--app-height` CSS variable from `visualViewport.height` via JS
- Offset fixed toolbar on iOS Safari using the layout/visual viewport delta

src/web/public/mobile.css

@@ -1903,6 +1903,14 @@ html.mobile-init .file-browser-panel {
   overscroll-behavior: none;
 }
 
+/* iPad Safari with tabs: position: fixed uses the layout viewport which extends
+   behind the browser chrome (tab bar). Calculate the offset between the layout
+   viewport (innerHeight) and the visual viewport (--app-height) to push the
+   toolbar into the visible area. --app-height is set by MobileDetection.updateAppHeight(). */
+.ios-device.safari-browser .toolbar {
+  bottom: calc(var(--safe-area-bottom) + (100vh - var(--app-height, 100vh)));
+}
+
 .ios-device.safari-browser .terminal-container {
   /* Keep momentum scrolling but prevent page bounce */
   -webkit-overflow-scrolling: touch;