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>

package.json

@@ -7,7 +7,7 @@
     "start": "node server/index.js",
     "dev": "concurrently \"npm run server\" \"npm run client\"",
     "server": "nodemon server/index.js",
-    "client": "cd client && npm start",
+    "client": "cd client && PORT=3456 npm start",
     "build": "cd client && npm run build",
     "build:commit": "npm run build && git add -f client/build/ && git commit -m \"Auto-commit built React app\" && git push",
     "install-all": "npm install && cd client && npm install"
@@ -22,7 +22,7 @@
   "license": "MIT",
   "dependencies": {
     "axios": "^1.6.2",
-    "better-sqlite3": "^9.2.2",
+    "better-sqlite3": "^13.0.3",
     "cors": "^2.8.5",
     "dotenv": "^16.3.1",
     "express": "^4.18.2",