Refactor USB screen, add debug logging, fix UI issues

- Rewrite UsbScreen to mirror ScannerScreen patterns (status bar,
  tap-to-connect port list, bottom FABs, SnackBar errors)
- Extract MeshCoreUsbManager from MeshCoreConnector for cleaner
  USB transport ownership
- Add debug logging throughout USB connection flow (connector,
  manager, web/native services)
- Print debug logs to console in debug mode even when app debug
  log setting is disabled
- Localize remaining hardcoded strings (Web Serial Device fallback
  label, USB status bar keys, companion firmware timeout hint)
- Fix Swedish misspelling in translations (stöderliga → stödda)
- Guard Linux notification init against missing D-Bus session bus
- Fix SNRIndicator hit-test error by adding minimum size constraints
- Update USB flow tests for new UI patterns

lib/l10n/app_en.arb

@@ -65,7 +65,19 @@
   "usbErrorAlreadyActive": "A USB connection is already active.",
   "usbErrorNoDeviceSelected": "No USB device was selected.",
   "usbErrorPortClosed": "The USB connection is not open.",
-  "usbErrorConnectTimedOut": "Timed out waiting for the device to respond.",
+  "usbErrorConnectTimedOut": "Connection timed out. Make sure the device has USB Companion firmware.",
+  "usbFallbackDeviceName": "Web Serial Device",
+  "usbStatus_notConnected": "Select a USB device",
+  "usbStatus_connecting": "Connecting to USB device...",
+  "usbStatus_searching": "Searching for USB devices...",
+  "usbConnectionFailed": "USB connection failed: {error}",
+  "@usbConnectionFailed": {
+    "placeholders": {
+      "error": {
+        "type": "String"
+      }
+    }
+  },
   "scanner_scanning": "Scanning for devices...",
   "scanner_connecting": "Connecting...",
   "scanner_disconnecting": "Disconnecting...",