fix(dm): clear error when contact not on device, log error_code detail

Device firmware requires contacts in its table to send DMs. Passing
a raw pubkey string results in error_code=2. Show actionable error
message instead of generic "Device error".

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

app/device_manager.py

@@ -827,14 +827,15 @@ class DeviceManager:
             return {'success': False, 'error': 'Device not connected'}
 
         try:
-            # Find contact dict from mc.contacts (used for retry/path info)
-            # send_msg() also accepts a hex pubkey string directly
+            # Find contact in device's contact table
             contact = self.mc.contacts.get(recipient_pubkey)
             if not contact:
                 contact = self.mc.get_contact_by_key_prefix(recipient_pubkey)
             if not contact:
-                # Use pubkey string directly — meshcore lib handles it
-                contact = recipient_pubkey
+                # Contact must exist on device to send DM
+                return {'success': False,
+                        'error': f'Contact not on device. '
+                                 f'Re-add {recipient_pubkey[:12]}... via Contacts page.'}
 
             # Generate timestamp once — same for all retries (enables receiver dedup)
             timestamp = int(time.time())