fix: standard ALTCHA PoW + lower default volume to 65%

Replace custom PoW scheme with standard ALTCHA: SHA-256(salt+number)
solver, base64-encoded {algorithm,challenge,number,salt,signature} verify
payload. Add verbose logging to diagnose future failures.
Lower default audio volume from 0.8 to 0.65.

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

src/main.js

@@ -133,7 +133,7 @@ window.addEventListener('load', () => {
     }
   });
 
-  S.audio.volume = 0.8;
+  S.audio.volume = 0.65;
 
   // Re-register MediaSession handlers when returning from background/screen-off/flip
   document.addEventListener('visibilitychange', () => {