Full health check: fix startup crash, add subnet input to network discovery

Server fixes:
- database.js: auto-create data/ directory on startup — was crashing on
  fresh clone because the gitignored directory didn't exist
- better-sqlite3 upgraded to latest (13.x) for Node 26 compatibility

Network discovery:
- discoverNetwork() accepts optional subnet param; scans that range instead
  of auto-detecting when provided
- New GET /api/network-subnets endpoint returns detected local interfaces
  so UI can offer quick-select buttons
- Discovery modal: subnet text input + Scan button + auto-detect quick-select;
  local subnet buttons populate once the modal is open
- server/index.js: pass subnet from request body to discoverNetwork()

Infra:
- React client runs on PORT=3456 in dev to avoid port conflicts
- .claude/launch.json added for one-command preview via Express (serves
  the pre-built client, no port conflict with the preview tool)

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

server/index.js

@@ -13,7 +13,7 @@ const notificationService = require('./notificationService');
 const cache = require('./cache');
 const security = require('./securityMiddleware');
 const metrics = require('./metrics');
-const { discoverNetwork } = require('./networkDiscovery');
+const { discoverNetwork, getLocalSubnets } = require('./networkDiscovery');
 const auditLog = require('./auditLog');
 
 // Initialize notifications table if it doesn't exist
@@ -1541,7 +1541,8 @@ app.post('/api/test-ntfy', async (req, res) => {
 // Network discovery — scan local network for monitorable hosts
 app.post('/api/network-discovery', async (req, res) => {
   try {
-    const hosts = await discoverNetwork();
+    const { subnet } = req.body || {};
+    const hosts = await discoverNetwork(subnet || null);
     res.json({ hosts });
   } catch (error) {
     console.error('Network discovery error:', error);
@@ -1549,6 +1550,15 @@ app.post('/api/network-discovery', async (req, res) => {
   }
 });
 
+// Return detected local subnets so the UI can offer them as quick-select options
+app.get('/api/network-subnets', (req, res) => {
+  try {
+    res.json({ subnets: getLocalSubnets() });
+  } catch (e) {
+    res.json({ subnets: [] });
+  }
+});
+
 // Clear cache endpoint for debugging
 app.post('/api/cache/clear', (req, res) => {
   cache.clear();