feat: F024 global AI assistant — contextual decision intelligence drawer

Wires the existing "AI Assistent" button in the TopBar and adds a 420px
slide-in drawer with context-aware suggestions, message thread, loading
animation, and action cards requiring manual confirmation. Template-based
mock service returns German-language answers keyed by route + question
keywords — no real LLM calls, no invented facts.

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

src/components/assistant/AssistantErrorState.tsx

@@ -0,0 +1,27 @@
+import { Alert, Box, Button } from '@mui/material'
+import { RefreshCw } from 'lucide-react'
+
+interface Props {
+  error: string
+  onRetry?: () => void
+}
+
+export function AssistantErrorState({ error, onRetry }: Props) {
+  return (
+    <Box sx={{ px: 2, py: 1 }}>
+      <Alert
+        severity="error"
+        sx={{ '& .MuiAlert-message': { fontSize: '0.8rem' } }}
+        action={
+          onRetry ? (
+            <Button size="small" onClick={onRetry} startIcon={<RefreshCw size={12} />} sx={{ textTransform: 'none', fontSize: '0.75rem' }}>
+              Erneut
+            </Button>
+          ) : undefined
+        }
+      >
+        {error}
+      </Alert>
+    </Box>
+  )
+}