feat: Add node discovery feature and improve advert notification

Implemented new "Discover Nodes" feature in Network Commands menu:
- Added .node_discover command to meshcli wrapper (cli.py)
- Created interactive modal with sortable table showing nearby repeaters
- Displays SNR, RSSI, path length, and signal quality indicators
- Added refresh functionality to rescan for nodes

Fixed advert notification to show clean "Advert sent" message
instead of full meshcli output.

Technical changes:
- app/meshcore/cli.py: Added node_discover() function with JSON parsing
- app/routes/api.py: Updated SPECIAL_COMMANDS and execute_special_command()
  to handle node_discover return type and clean advert message
- app/templates/base.html: Added "Discover Nodes" menu button and modal
- app/static/js/app.js: Added discoverNodes() and displayNodeDiscoveryResults()
- README.md: Added documentation for Discover Nodes feature

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app/meshcore/cli.py

@@ -368,6 +368,47 @@ def floodadv() -> Tuple[bool, str]:
     return success, stdout or stderr
 
 
+def node_discover() -> Tuple[bool, List[Dict]]:
+    """
+    Discover nearby mesh nodes (repeaters).
+
+    Uses .node_discover command which returns JSON array of nearby repeaters
+    with SNR, RSSI, and other metadata.
+
+    Returns:
+        Tuple of (success, nodes_list)
+        Each node dict contains:
+        {
+            'SNR': float,
+            'RSSI': int,
+            'path_len': int,
+            'node_type': int (2=REP),
+            'SNR_in': float,
+            'tag': str (hex),
+            'pubkey': str (hex)
+        }
+    """
+    success, stdout, stderr = _run_command(['.node_discover'])
+
+    if not success:
+        logger.error(f"node_discover failed: {stderr}")
+        return False, []
+
+    try:
+        # Parse JSON array from stdout
+        nodes = json.loads(stdout)
+        if not isinstance(nodes, list):
+            logger.error(f"node_discover returned non-array: {stdout}")
+            return False, []
+
+        logger.info(f"Discovered {len(nodes)} nearby nodes")
+        return True, nodes
+
+    except json.JSONDecodeError as e:
+        logger.error(f"Failed to parse node_discover JSON: {e}, output: {stdout}")
+        return False, []
+
+
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