Phase 2: multitouch

Panel used onClick, which cannot express a chord at all - so hold mode,
which is the persisted default answering mode, was unusable. It now
tracks pointers per pointerId and reports onPress per control plus
onGestureEnd when the last finger lifts.

The split is deliberate: Panel owns pointer bookkeeping because it owns
the DOM, and App keeps deciding what a chord means. Hold mode grades on
onGestureEnd, tap mode accumulates presses and waits for Check, and
Explore still selects on press. Both map onto reducer actions that
already existed.

Two details carried over from the reference that are easy to delete by
accident, and are now written down:

- touch-action: none on the panel. Without it the browser claims the
  second finger as a pinch or a pan and no chord can ever form.
- pointerup is heard on the window, not the button, because fingers
  routinely lift outside the key they went down on.

Keyboard still works: Enter or Space presses and ends the gesture.

141 tests, typecheck clean, touch-action confirmed in the built CSS, and
the panel renders unchanged. Actual multi-finger behaviour cannot be
verified headlessly - that needs the phone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ui/Panel.module.css

@@ -8,6 +8,17 @@
   box-shadow: var(--shadow);
   display: flex;
   flex-direction: column;
+  /* Without this the browser claims a second finger as a pinch or a pan, and a
+     chord can never be formed. This is what makes multitouch work at all. */
+  touch-action: none;
+  user-select: none;
+  -webkit-user-select: none;
+  -webkit-touch-callout: none;
+}
+
+/* Pressed, whether or not it is lit. */
+.down {
+  transform: scale(0.95);
 }
 
 /* ---- shared key look ---- */