Fix audio timeout when changing buffer size/sample rate

Two bugs:

1. **Deadlock in set_audio_profile() (root cause of timeout)**
   jack_client.stop() was called BEFORE killing the JACK server.
   jack.Client.deactivate() is a synchronous call that blocks until
   the JACK RT callback thread acknowledges. When ALSA reconfigures
   (period/rate change), the RT thread can stall and deactivate()
   hangs permanently. Reversed order: kill JACK server first (via
   killall jackd) to force-kill the RT thread, then clean up the
   Python client. Same fix applied to device hot-swap path.

2. **Pipeline crash after period change**
   _CombFilter.buf and _AllpassFilter.buf were fixed-size arrays
   at BLOCK_SIZE (256). If JACK restarts with a different period
   (e.g. 512), self.buf[:] = block throws a shape mismatch in the
   RT callback -> audio dies permanently. Made internal buffers
   dynamically resize to match input block size.

src/web/server.py

@@ -1480,17 +1480,18 @@ class WebServer:
                 target_profile["period"], target_profile["rate"],
             )
 
-            # Stop JACK audio client first
             jack_client = self.deps.jack_audio
-            if jack_client:
-                jack_client.stop()
 
             # Update LATENCY_PROFILES with custom entry so latency_profile resolves it
             LATENCY_PROFILES[effective_key] = target_profile
 
             # ── Kill JACK server FIRST (breaks any client deadlock) ──
-            # This interrupts the audio callback thread, releasing any lock
-            # that jack_client.stop() would need.
+            # jack.Client.deactivate() is a synchronous JACK protocol call
+            # that blocks until the RT callback thread acknowledges. If the
+            # RT thread is stuck (ALSA reconfiguring, xruns), deactivate()
+            # hangs permanently.  Kill the server first to force-kill the
+            # RT thread, then clean up the client.
+            # See also: JackAudioClient.stop() which calls deactivate().
             try:
                 audio_sys.stop_jack()
             except Exception:
@@ -1634,13 +1635,15 @@ class WebServer:
                 try:
                     audio_sys = self.deps.audio_system
                     jack_client = self.deps.jack_audio
-                    if jack_client:
-                        jack_client.stop()
+                    # Kill JACK server FIRST to avoid deadlock on
+                    # jack.Client.deactivate() (same pattern as profile switch)
                     if "input_device" in data:
                         audio_sys.config.input_device = str(data["input_device"])
                     if "output_device" in data:
                         audio_sys.config.output_device = str(data["output_device"])
                     audio_sys.stop_jack()
+                    if jack_client:
+                        jack_client.stop()
                     ok = audio_sys.start_jack(timeout=10)
                     if not ok:
                         logger.warning("JACK restart failed after device change — rolling back")