fix: correct relative imports in proxmox subpackage and CSP headers

- Fix all `from ..` → `from ...` in app/api/proxmox/*.py after the
  proxmox.py split (PR #6) placed modules one level deeper, causing
  ModuleNotFoundError on startup
- Broaden Content-Security-Policy to allow CDN sources used in templates:
  cdnjs.cloudflare.com (Font Awesome), cdn.jsdelivr.net (Bootstrap,
  Chart.js, xterm), fonts.googleapis.com/gstatic.com (Google Fonts),
  esm.sh (noVNC) — the overly-strict CSP was blocking all icons and
  charts from loading in the browser

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

backend/app/main.py

@@ -178,11 +178,11 @@ def create_app() -> FastAPI:
         response.headers["Referrer-Policy"] = "strict-origin-when-cross-origin"
         response.headers["Content-Security-Policy"] = (
             "default-src 'self'; "
-            "script-src 'self' 'unsafe-inline' 'unsafe-eval'; "
-            "style-src 'self' 'unsafe-inline'; "
+            "script-src 'self' 'unsafe-inline' 'unsafe-eval' https://cdn.jsdelivr.net https://esm.sh; "
+            "style-src 'self' 'unsafe-inline' https://cdnjs.cloudflare.com https://cdn.jsdelivr.net https://fonts.googleapis.com; "
             "img-src 'self' data: blob:; "
-            "connect-src 'self' wss: ws:; "
-            "font-src 'self' data:; "
+            "connect-src 'self' wss: ws: https://esm.sh; "
+            "font-src 'self' data: https://cdnjs.cloudflare.com https://fonts.gstatic.com; "
             "frame-ancestors 'self';"
         )
         if not settings.DEBUG: