Sync NAM engine block_size with JACK period on profile switch

- Add FastNAMHost.set_block_size() — reloads current model with
  new block_size, keeping the engine in sync with JACK's period
- Wire into POST /api/audio/profile so NAM engine restarts with
  correct frame count when switching between standard/low/stable
- Prevents sample-alignment drift when JACK period changes but
  NAM engine stays at old block_size (was causing passthrough)

src/web/server.py

@@ -1361,6 +1361,10 @@ class WebServer:
             # Restart JACK audio client
             if jack_client:
                 jack_client.start()
+            # Sync NAM engine block size with new period
+            nam_host = self.deps.nam_host
+            if nam_host and hasattr(nam_host, 'set_block_size'):
+                nam_host.set_block_size(profile["period"])
             await self._manager.broadcast({
                 "type": "audio_profile_changed",
                 "profile": profile_key,