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/composables/useNostr.ts

@@ -112,6 +112,17 @@ export function useNostr() {
   const isLoggedIn = computed(() => !!pubkey.value && !!bot.value)
   const hasExtension = computed(() => !!window.nostr)
 
+  /** Wait for window.nostr to appear (mobile signers inject late) */
+  async function waitForSigner(timeoutMs = 3000): Promise<boolean> {
+    if (window.nostr) return true
+    const start = Date.now()
+    while (Date.now() - start < timeoutMs) {
+      await new Promise(r => setTimeout(r, 200))
+      if (window.nostr) return true
+    }
+    return false
+  }
+
   /**
    * Authenticate with the server using NIP-98 signed request.
    * Works with NIP-07 extension, Amber signer, or local nsec.
@@ -159,17 +170,21 @@ export function useNostr() {
    */
   async function login(): Promise<{ pubkey: string; bot: BotData | null }> {
     if (!window.nostr) {
-      // Fall back to stored nsec if available
-      const storedNsec = localStorage.getItem('bf_nsec')
-      if (storedNsec) {
-        return loginWithNsec(storedNsec)
+      // Mobile signers (Amber) inject window.nostr late — poll for up to 3s
+      const found = await waitForSigner(3000)
+      if (!found) {
+        // Fall back to stored nsec if available
+        const storedNsec = localStorage.getItem('bf_nsec')
+        if (storedNsec) {
+          return loginWithNsec(storedNsec)
+        }
+        throw new Error('No Nostr signer found. Install a browser extension or use "Generate New Identity".')
       }
-      throw new Error('No Nostr signer found. Install a browser extension or use "Generate New Identity".')
     }
 
     // Verify extension is responsive
     try {
-      await window.nostr.getPublicKey()
+      await window.nostr!.getPublicKey()
     } catch {
       throw new Error('Nostr signer denied access. Approve the request and try again.')
     }
@@ -419,6 +434,7 @@ export function useNostr() {
     hasExtension,
     hasStoredKey,
     login,
+    waitForSigner,
     generateLogin,
     loginWithNsec,
     registerBot,