feat(legal): imprint & privacy notices, "Über MindSift"

- LEGAL_IMPRINT_/LEGAL_PRIVACY_ URL and Markdown (inline or *_FILE);
  only http(s) URLs accepted
- /auth/status exposes legal info; public GET /legal/{imprint|privacy}
- Links on login/setup screen and in the settings footer: URL opens a new
  tab, Markdown a reading modal; hidden when nothing is configured
- "Über MindSift" modal (version, license, repository) in settings and help
- Docs and compose/env examples

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/components/ReadingModal.tsx

@@ -0,0 +1,37 @@
+import React, { useEffect } from 'react';
+import { X } from 'lucide-react';
+
+interface ReadingModalProps {
+  title: string;
+  onClose: () => void;
+  children: React.ReactNode;
+}
+
+// Slim modal for reading text (legal notices, about); Escape or backdrop click closes it
+export const ReadingModal: React.FC<ReadingModalProps> = ({ title, onClose, children }) => {
+  useEffect(() => {
+    const onKey = (e: KeyboardEvent) => e.key === 'Escape' && onClose();
+    window.addEventListener('keydown', onKey);
+    return () => window.removeEventListener('keydown', onKey);
+  }, [onClose]);
+
+  return (
+    <div
+      className="fixed inset-0 z-50 flex items-center justify-center p-4 bg-black/60 backdrop-blur-xs"
+      onClick={(e) => e.target === e.currentTarget && onClose()}
+      role="dialog"
+      aria-modal="true"
+      aria-label={title}
+    >
+      <div className="panel-surface w-full max-w-2xl max-h-[85vh] flex flex-col shadow-panel">
+        <div className="flex items-center justify-between px-4 py-3 border-b border-border">
+          <h2 className="text-sm font-bold text-main">{title}</h2>
+          <button type="button" onClick={onClose} className="btn-icon" title="Schließen">
+            <X className="w-4 h-4" />
+          </button>
+        </div>
+        <div className="p-4 overflow-y-auto text-sm text-main">{children}</div>
+      </div>
+    </div>
+  );
+};