Add device reservation to DMR mode and improve USB busy error message

DMR was missing checkDeviceAvailability/reserveDevice/releaseDevice
calls that other modes (SSTV, listening post) use, so the device
dropdown showed device 0 as available even when another process held
it. Also detect USB claim errors from rtl_fm and surface a clear
message telling the user to pick a different device.

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

routes/dmr.py

@@ -334,11 +334,14 @@ def start_dmr() -> Response:
             if dmr_active_device is not None:
                 app_module.release_sdr_device(dmr_active_device)
                 dmr_active_device = None
-            # Surface the most relevant error to the user
+            # Surface a clear error to the user
             detail = rtl_err.strip() or dsd_err.strip()
-            msg = 'Failed to start DSD pipeline'
+            if 'usb_claim_interface' in rtl_err or 'Failed to open' in rtl_err:
-            if detail:
+                msg = f'SDR device {device} is busy — it may be in use by another mode or process. Try a different device.'
-                msg += f': {detail}'
+            elif detail:
+                msg = f'Failed to start DSD pipeline: {detail}'
+            else:
+                msg = 'Failed to start DSD pipeline'
             return jsonify({'status': 'error', 'message': msg}), 500
 
         # Drain rtl_fm stderr in background to prevent pipe blocking

static/js/modes/dmr.js

@@ -59,6 +59,11 @@ function startDmr() {
     const gain = parseInt(document.getElementById('dmrGain')?.value || 40);
     const device = typeof getSelectedDevice === 'function' ? getSelectedDevice() : 0;
 
+    // Check device availability before starting
+    if (typeof checkDeviceAvailability === 'function' && !checkDeviceAvailability('dmr')) {
+        return;
+    }
+
     fetch('/dmr/start', {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
@@ -80,6 +85,9 @@ function startDmr() {
             updateDmrSynthStatus();
             const statusEl = document.getElementById('dmrStatus');
             if (statusEl) statusEl.textContent = 'DECODING';
+            if (typeof reserveDevice === 'function') {
+                reserveDevice(parseInt(device), 'dmr');
+            }
             if (typeof showNotification === 'function') {
                 showNotification('DMR', `Decoding ${frequency} MHz (${protocol.toUpperCase()})`);
             }
@@ -104,6 +112,9 @@ function stopDmr() {
         updateDmrSynthStatus();
         const statusEl = document.getElementById('dmrStatus');
         if (statusEl) statusEl.textContent = 'STOPPED';
+        if (typeof releaseDevice === 'function') {
+            releaseDevice('dmr');
+        }
     })
     .catch(err => console.error('[DMR] Stop error:', err));
 }