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/index.html

@@ -1 +1 @@
-<!doctype html><html lang="en" data-theme="dark"><head><meta charset="utf-8"/><link rel="icon" type="image/png" href="/app-icon.png"/><meta name="viewport" content="width=device-width,initial-scale=1"/><meta name="theme-color" content="#000000"/><meta name="description" content="SkyWatch - Infrastructure monitoring and alerting system"/><link rel="apple-touch-icon" href="/app-icon.png"/><link rel="manifest" href="/manifest.json"/><title>SkyWatch</title><script defer="defer" src="/static/js/main.f56b4907.js"></script><link href="/static/css/main.d4111563.css" rel="stylesheet"></head><body><noscript>You need to enable JavaScript to run this app.</noscript><div id="root"></div></body></html>
+<!doctype html><html lang="en" data-theme="dark"><head><meta charset="utf-8"/><link rel="icon" type="image/png" href="/app-icon.png"/><meta name="viewport" content="width=device-width,initial-scale=1"/><meta name="theme-color" content="#000000"/><meta name="description" content="SkyWatch - Infrastructure monitoring and alerting system"/><link rel="apple-touch-icon" href="/app-icon.png"/><link rel="manifest" href="/manifest.json"/><title>SkyWatch</title><script defer="defer" src="/static/js/main.ee340b3d.js"></script><link href="/static/css/main.d4111563.css" rel="stylesheet"></head><body><noscript>You need to enable JavaScript to run this app.</noscript><div id="root"></div></body></html>