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.ts

@@ -0,0 +1,33 @@
+/**
+ * `KeyboardEvent.code` values the push-to-talk / push-to-deafen rebind must
+ * never accept. Binding one of these turns it into a keyboard trap (the call
+ * hotkey listener swallows the key everywhere outside an editable field for
+ * the rest of the call) or collides with a bare modifier chord.
+ */
+const UNBINDABLE_CALL_KEY_CODES = new Set<string>([
+  'Escape',
+  'Tab',
+  'Enter',
+  'NumpadEnter',
+  'ArrowUp',
+  'ArrowDown',
+  'ArrowLeft',
+  'ArrowRight',
+  'Home',
+  'End',
+  'PageUp',
+  'PageDown',
+  'ShiftLeft',
+  'ShiftRight',
+  'ControlLeft',
+  'ControlRight',
+  'AltLeft',
+  'AltRight',
+  'MetaLeft',
+  'MetaRight',
+]);
+
+/** Whether `code` is safe to bind as a call hotkey (PTT / push-to-deafen). */
+export function isBindableCallKey(code: string): boolean {
+  return code.length > 0 && !UNBINDABLE_CALL_KEY_CODES.has(code);
+}