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/database.js

@@ -1,7 +1,11 @@
 const Database = require('better-sqlite3');
 const path = require('path');
+const fs = require('fs');
 
-const db = new Database(path.join(__dirname, '../data/monitor.db'));
+const dataDir = path.join(__dirname, '../data');
+if (!fs.existsSync(dataDir)) fs.mkdirSync(dataDir, { recursive: true });
+
+const db = new Database(path.join(dataDir, 'monitor.db'));
 
 // Create tables
 db.exec(`