fix: CPU calc, disk card, lifespan, remove Google Fonts CDN, fix doc link

- Fix Docker container CPU % (multiply by num_cpus)
- Add missing Disk usage card to system status grid
- Replace deprecated @app.on_event with lifespan context manager
- Move `import requests` to module level
- Remove Google Fonts CDN dependency from CSS
- Update naming doc URL to real raw Gitea link

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/css/style.css

@@ -1,5 +1,3 @@
-@import url('https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&display=swap');
-
 :root {
     /* Colors inspired by the mock-up */
     --bg-main: #fbfbf9;