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>

client/build/asset-manifest.json

@@ -1,15 +1,15 @@
 {
   "files": {
     "main.css": "/static/css/main.d4111563.css",
-    "main.js": "/static/js/main.f56b4907.js",
+    "main.js": "/static/js/main.ee340b3d.js",
     "static/js/453.d456ce70.chunk.js": "/static/js/453.d456ce70.chunk.js",
     "index.html": "/index.html",
     "main.d4111563.css.map": "/static/css/main.d4111563.css.map",
-    "main.f56b4907.js.map": "/static/js/main.f56b4907.js.map",
+    "main.ee340b3d.js.map": "/static/js/main.ee340b3d.js.map",
     "453.d456ce70.chunk.js.map": "/static/js/453.d456ce70.chunk.js.map"
   },
   "entrypoints": [
     "static/css/main.d4111563.css",
-    "static/js/main.f56b4907.js"
+    "static/js/main.ee340b3d.js"
   ]
 }