migrate dashboard to real HeroUI v3 components and neutral theme

The frontend was built against HeroUI v2 props/classes (variant="light"/"flat",
color="primary", startContent, text-tiny, <Avatar src=/name=>) while
@heroui/react ^3.2.1 is installed, so most styling was silently ignored.
Rewrites every page/component onto the actual v3 API (variant/color enums,
Tooltip/Dropdown trigger-content structure, Avatar.Image/Fallback via a new
AppAvatar helper) and replaces the custom orange branding with HeroUI's
neutral default theme tokens. Also compacts the dashboard stat tiles, narrows
the sidebar, and swaps the activity stat grid for a single-hue comparison bar
chart.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/shared/ErrorState.tsx

@@ -8,14 +8,14 @@ type Props = {
 
 export function ErrorState({ message = 'Ein Fehler ist aufgetreten', onRetry }: Props) {
   return (
-    <Card className="border border-danger-100/30 bg-danger-50/10">
+    <Card className="border border-danger-soft bg-danger-soft/40">
       <CardContent className="flex flex-col items-center gap-3 py-8">
-        <div className="flex size-12 items-center justify-center rounded-full bg-danger-500/10 text-danger-400">
+        <div className="flex size-12 items-center justify-center rounded-full bg-danger-soft text-danger">
           <AlertTriangle size={24} />
         </div>
-        <p className="text-small text-default-500">{message}</p>
+        <p className="text-sm text-muted">{message}</p>
         {onRetry && (
-          <button className="flex items-center gap-1 text-tiny text-primary-400 hover:text-primary-300 transition-colors" onClick={onRetry}>
+          <button className="flex items-center gap-1 text-xs text-accent hover:opacity-80 transition-opacity" onClick={onRetry}>
             <RefreshCw size={12} /> Erneut versuchen
           </button>
         )}