fix(calls): hotkey rebind denylist; ignore modifiers; keep Space on buttons

- useKeyBind refuses Tab/Enter/arrows/Home/End/Page*/Escape and bare
  modifier codes, and refuses a code equal to the other call key, with an
  inline message (isBindableCallKey, unit-tested).
- PTT and deafen handlers ignore events with Ctrl/Alt/Meta held (deafen
  also Shift), so Cmd+M / Ctrl+M no longer toggle deafen.
- PTT only preventDefault()s when the target is not an interactive
  control, so Space still activates focused buttons during a call.

Fixes #23

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/utils/callKeybind.test.ts

@@ -0,0 +1,47 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { isBindableCallKey } from './callKeybind';
+
+test('isBindableCallKey rejects navigation-critical codes', () => {
+  [
+    'Escape',
+    'Tab',
+    'Enter',
+    'NumpadEnter',
+    'ArrowUp',
+    'ArrowDown',
+    'ArrowLeft',
+    'ArrowRight',
+    'Home',
+    'End',
+    'PageUp',
+    'PageDown',
+  ].forEach((code) => {
+    assert.equal(isBindableCallKey(code), false, `${code} should be unbindable`);
+  });
+});
+
+test('isBindableCallKey rejects bare modifier codes', () => {
+  [
+    'ShiftLeft',
+    'ShiftRight',
+    'ControlLeft',
+    'ControlRight',
+    'AltLeft',
+    'AltRight',
+    'MetaLeft',
+    'MetaRight',
+  ].forEach((code) => {
+    assert.equal(isBindableCallKey(code), false, `${code} should be unbindable`);
+  });
+});
+
+test('isBindableCallKey accepts ordinary keys', () => {
+  ['Space', 'KeyM', 'KeyQ', 'Digit1', 'F13'].forEach((code) => {
+    assert.equal(isBindableCallKey(code), true, `${code} should be bindable`);
+  });
+});
+
+test('isBindableCallKey rejects the empty string', () => {
+  assert.equal(isBindableCallKey(''), false);
+});