Add server health endpoint and status page

Backend:
- New /api/server/health endpoint returning server, paths, disk,
  mods, steamcmd, and frontend status
- Health endpoint checks directory existence/writability, disk usage,
  server binary, and frontend dist availability

Frontend:
- New Status page (route /status) with sections for Server, SteamCMD,
  Mods, Frontend, Directories (with exists/writable badges), and
  Disk Usage (with size formatting and used-percent coloring)
- Paths are clickable to copy and use start-truncation (ellipsis on
  the left) with native hover tooltip
- Health API client integration with 10s auto-refresh

Misc:
- Update .gitignore and Makefile, minor Layout nav addition

frontend/src/components/Layout.tsx

@@ -8,6 +8,7 @@ const nav = [
   { to: '/modlists', label: 'Modlists', icon: '📦' },
   { to: '/mods', label: 'Mods', icon: '🗂' },
   { to: '/logs', label: 'Logs', icon: '📋' },
+  { to: '/status', label: 'Status', icon: '📊' },
 ]
 
 export default function Layout({ children }: { children: ReactNode }) {