fix(maestro): hide mobile key bar when keyboard is dismissed while focused

Focus alone was the wrong signal: dismissing the soft keyboard (e.g. Android
back button) keeps the terminal focused, so the key bar lingered. Detect the
keyboard's actual visibility from the visual-viewport shrink — while an editable
element is focused, a height past a threshold below the focus-free baseline
means the keyboard is up — and gate the bar on a new `html.kb-open` class in
addition to the per-terminal focus class.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/styles.css

@@ -1192,10 +1192,13 @@ textarea {
   background: var(--surface);
 }
 
-/* Shown on touch only while the terminal is focused (soft keyboard up) — the
-   `keybar-active` class is toggled on textarea focus/blur in attach.ts. */
+/* Shown on touch only while *this* terminal is focused (`keybar-active`, toggled
+   on textarea focus/blur in attach.ts) AND the soft keyboard is actually up
+   (`html.kb-open`, detected from the visual-viewport shrink in viewport.ts).
+   Both are needed: dismissing the keyboard doesn't blur the terminal, so focus
+   alone would leave the bar lingering. */
 @media (pointer: coarse) {
-  .attach-keybar.keybar-active {
+  html.kb-open .attach-keybar.keybar-active {
     display: grid;
   }
 }