perf(D): defer maplibre-gl CSS into the MapContainer chunk (#4002)

Closes #3991. The 70 KiB MapLibre stylesheet was statically imported at the
top of src/main.ts and 98% unused on first paint — it still render-blocked
FCP. Moving the import to src/components/MapContainer.ts (only reached via
the dynamic import at src/app/panel-layout.ts:905) lets Vite associate the
CSS with the lazy chunk so it loads when MapContainer mounts.

Deterministic signal: maplibre-B46vZ9Yf.css drops from dist/index.html's
initial <link rel="stylesheet"> set (3 → 2 CSS files).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>