fix: mobile nostr signer detection, mobile TTS auto-unlock, button loaders

- Nostr signer: poll for window.nostr up to 3s on mobile (Amber injects
  late). Both login() and handleSignerLogin() now wait before failing.
- Mobile TTS: install global one-time click/touch/keydown handler to
  auto-unlock AudioContext when fight pages mount. Previously only
  triggered by explicit sound toggle, so mobile TTS silently failed.
- Add loading spinners to "I BUILD BOTS" and "I FIGHT MYSELF" buttons.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/game/audio/index.ts

@@ -16,6 +16,27 @@ export function stopAllAudio() {
   resetGainNodes()
 }
 
+// === CROSS-CUTTING: auto-unlock AudioContext on first user gesture ===
+// Mobile browsers require a user gesture to unlock AudioContext. Instead of
+// relying solely on the sound toggle button, install a one-time listener on
+// the first click/tap/keydown anywhere on the page.
+
+let _autoUnlockInstalled = false
+
+export function installAutoUnlock() {
+  if (_autoUnlockInstalled) return
+  _autoUnlockInstalled = true
+  const handler = () => {
+    ensureAudioContext()
+    document.removeEventListener('click', handler, true)
+    document.removeEventListener('touchstart', handler, true)
+    document.removeEventListener('keydown', handler, true)
+  }
+  document.addEventListener('click', handler, true)
+  document.addEventListener('touchstart', handler, true)
+  document.addEventListener('keydown', handler, true)
+}
+
 // === CROSS-CUTTING: ensureAudioContext ===
 // Must be called from user gesture (click/tap) to unlock audio on mobile browsers