fix: comprehensive dark mode support across all pages

- Fix body/html dark background to surface-950
- Add dark scrollbar styling
- Fix App.svelte wrapper to use Tailwind dark: classes instead of conditional strings
- Add dark:bg-surface-800 and dark:border-surface-700 to all cards in Dashboard, Docker, Files, Gitea
- Add dark text variants for headings, labels, and content text
- Add dark hover states for interactive elements
- Enable cross-category sidebar drag-and-drop

dashboard/frontend/src/app.css

@@ -22,6 +22,7 @@
   --color-surface-700: #334155;
   --color-surface-800: #1e293b;
   --color-surface-900: #0f172a;
+  --color-surface-950: #020617;
 
   --color-emerald-400: #34d399;
   --color-emerald-500: #10b981;
@@ -57,8 +58,10 @@ body {
 ::-webkit-scrollbar-thumb:hover { background: var(--color-surface-400); }
 
 /* Dark mode overrides */
-.dark body,
-.dark {
-  background: var(--color-surface-900);
+html.dark body {
+  background: var(--color-surface-950);
   color: var(--color-surface-200);
 }
+
+html.dark ::-webkit-scrollbar-thumb { background: var(--color-surface-600); }
+html.dark ::-webkit-scrollbar-thumb:hover { background: var(--color-surface-500); }