feat: cell-to-cell (PIC mesh) connection feature

Site-to-site WireGuard tunnels between PIC cells with automatic DNS forwarding.
Each cell generates an invite JSON (public key, endpoint, VPN subnet, DNS IP,
domain); the remote cell imports it to establish a bidirectional tunnel and
CoreDNS forwarding block so each cell's domain resolves across the mesh.

Backend:
- CellLinkManager: invite generation, add/remove connections, live WireGuard
  handshake status; stores links in data/cell_links.json
- WireGuardManager: add_cell_peer() accepts subnet CIDRs (not /32) and an
  optional endpoint for site-to-site peers; _read_iface_field() reads port,
  address, and network directly from wg0.conf at runtime instead of constants
- NetworkManager: add/remove CoreDNS forwarding blocks per remote cell domain
- app.py: /api/cells/* routes; _next_peer_ip() derives VPN range from
  configured address so peer allocation follows any address change

Frontend:
- CellNetwork page: invite panel (JSON + QR), connect form (paste JSON),
  connected cells list (green/red status, disconnect button)
- App.jsx: Cell Network nav entry and route

Tests: 25 new tests across test_wireguard_manager, test_network_manager,
test_cell_link_manager (263 total)

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

api/cell_link_manager.py

@@ -0,0 +1,122 @@
+#!/usr/bin/env python3
+"""
+CellLinkManager — manages site-to-site connections between PIC cells.
+
+Each connection is stored in data/cell_links.json and manifests as:
+  - A WireGuard [Peer] block (AllowedIPs = remote cell's VPN subnet)
+  - A CoreDNS forwarding block (remote domain → remote cell's DNS IP)
+"""
+
+import os
+import json
+import logging
+from datetime import datetime
+from typing import Any, Dict, List, Optional
+
+logger = logging.getLogger(__name__)
+
+
+class CellLinkManager:
+    def __init__(self, data_dir: str, config_dir: str, wireguard_manager, network_manager):
+        self.data_dir = data_dir
+        self.config_dir = config_dir
+        self.wireguard_manager = wireguard_manager
+        self.network_manager = network_manager
+        self.links_file = os.path.join(data_dir, 'cell_links.json')
+
+    # ── Storage ───────────────────────────────────────────────────────────────
+
+    def _load(self) -> List[Dict[str, Any]]:
+        if os.path.exists(self.links_file):
+            try:
+                with open(self.links_file) as f:
+                    return json.load(f)
+            except Exception:
+                return []
+        return []
+
+    def _save(self, links: List[Dict[str, Any]]):
+        with open(self.links_file, 'w') as f:
+            json.dump(links, f, indent=2)
+
+    # ── Public API ────────────────────────────────────────────────────────────
+
+    def generate_invite(self, cell_name: str, domain: str) -> Dict[str, Any]:
+        """Return an invite package describing this cell for another cell to import."""
+        keys = self.wireguard_manager.get_keys()
+        srv = self.wireguard_manager.get_server_config()
+        server_vpn_ip = self.wireguard_manager._get_configured_address().split('/')[0]
+        return {
+            'cell_name': cell_name,
+            'public_key': keys['public_key'],
+            'endpoint': srv.get('endpoint'),
+            'vpn_subnet': self.wireguard_manager._get_configured_network(),
+            'dns_ip': server_vpn_ip,
+            'domain': domain,
+            'version': 1,
+        }
+
+    def list_connections(self) -> List[Dict[str, Any]]:
+        return self._load()
+
+    def add_connection(self, invite: Dict[str, Any]) -> Dict[str, Any]:
+        """Import a remote cell's invite and establish the connection."""
+        links = self._load()
+        name = invite['cell_name']
+        if any(l['cell_name'] == name for l in links):
+            raise ValueError(f"Cell '{name}' is already connected")
+
+        ok = self.wireguard_manager.add_cell_peer(
+            name=name,
+            public_key=invite['public_key'],
+            endpoint=invite.get('endpoint', ''),
+            vpn_subnet=invite['vpn_subnet'],
+        )
+        if not ok:
+            raise RuntimeError(f"Failed to add WireGuard peer for cell '{name}'")
+
+        dns_result = self.network_manager.add_cell_dns_forward(
+            domain=invite['domain'],
+            dns_ip=invite['dns_ip'],
+        )
+        if dns_result.get('warnings'):
+            logger.warning('DNS forward warnings for %s: %s', name, dns_result['warnings'])
+
+        link = {
+            'cell_name': name,
+            'public_key': invite['public_key'],
+            'endpoint': invite.get('endpoint'),
+            'vpn_subnet': invite['vpn_subnet'],
+            'dns_ip': invite['dns_ip'],
+            'domain': invite['domain'],
+            'connected_at': datetime.utcnow().isoformat(),
+        }
+        links.append(link)
+        self._save(links)
+        return link
+
+    def remove_connection(self, cell_name: str):
+        """Tear down a cell connection by name."""
+        links = self._load()
+        link = next((l for l in links if l['cell_name'] == cell_name), None)
+        if not link:
+            raise ValueError(f"Cell '{cell_name}' not found")
+
+        self.wireguard_manager.remove_peer(link['public_key'])
+        self.network_manager.remove_cell_dns_forward(link['domain'])
+
+        links = [l for l in links if l['cell_name'] != cell_name]
+        self._save(links)
+
+    def get_connection_status(self, cell_name: str) -> Dict[str, Any]:
+        """Return link record enriched with live WireGuard handshake status."""
+        links = self._load()
+        link = next((l for l in links if l['cell_name'] == cell_name), None)
+        if not link:
+            raise ValueError(f"Cell '{cell_name}' not found")
+        try:
+            st = self.wireguard_manager.get_peer_status(link['public_key'])
+            return {**link, 'online': st.get('online', False),
+                    'last_handshake': st.get('last_handshake')}
+        except Exception:
+            return {**link, 'online': False, 'last_handshake': None}