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>

.claude/launch.json

@@ -0,0 +1,12 @@
+{
+  "version": "0.0.1",
+  "configurations": [
+    {
+      "name": "SkyWatch",
+      "runtimeExecutable": "node",
+      "runtimeArgs": ["server/index.js"],
+      "port": 3001,
+      "url": "http://localhost:3001"
+    }
+  ]
+}