Add ThemeContext (dark/light, persisted to localStorage, applied as
data-theme on <html>). Theme toggle is a sun/moon flip switch at the
right end of the dashboard info bar.
CSS overrides in index.css target every hardcoded Tailwind arbitrary
colour class used across all pages via [data-theme="light"] selectors,
so the entire panel switches without touching individual page files.
Light palette: page #f0f4f8, cards/sidebar #ffffff, hover #f1f5f9,
borders #e2e8f0, text #0f172a. Chart tooltip is made theme-aware via
inline-style interpolation since Tailwind CSS overrides cannot reach
Recharts inline styles.
Adds a compact info strip at the top of the NixPanel dashboard showing
Username, Hostname, OS, NixPanel Version, Load Averages (live via
WebSocket), and Server Monitoring alert count. Alert count is derived
from GET /api/nixserver/services — green "All Clear" when all services
are running, amber "N Alerts" for any stopped/failed service.
Also fixes the version footer error state in Layout.tsx (versionError
→ "Version unavailable" fallback with retry button) so the spinner no
longer hangs indefinitely when the version API is unreachable.
NixReaper