fix: clean stale JACK SHM before client creation in profile handler

jack.Client('pi-multifx') fails with server_error (0x21) when stale
jack_sem.0_default_pi-multifx segments remain in /dev/shm from a
previous client instance. This commonly happens after a profile change
(buffer+rate) because:

1. stop_jack() kills jackd and cleans SHM
2. start_jack() starts new JACK server
3. Old process's JackAudioClient.__del__ (garbage collection) recreates
   the pi-multifx semaphore in /dev/shm
4. jack_client.start() → jack.Client('pi-multifx') finds stale SHM
   and fails with server_error → no pi-multifx ports → NAM never
   called → 0% nam_cpu → 'nam is dying'

Fix:
- Clean /dev/shm/jack* in profile handler right before jack_client.start()
- JackAudioClient.start() also retries once with SHM cleanup on
  server_error, so other callers are also protected.

src/web/server.py

@@ -1748,6 +1748,20 @@ class WebServer:
                     target_profile["rate"],
                 )
 
+            # Clean stale client SHM before creating new JACK client.
+            # The old process's jack.Client __del__ can recreate the
+            # pi-multifx semaphore after stop_jack() cleaned it.
+            try:
+                from pathlib import Path
+                import shutil
+                for p in Path("/dev/shm").glob("jack*"):
+                    if p.is_dir():
+                        shutil.rmtree(p, ignore_errors=True)
+                    else:
+                        p.unlink(missing_ok=True)
+            except Exception:
+                pass
+
             # Restart JACK audio client
             if jack_client:
                 try: