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/lib/api.ts

@@ -141,6 +141,12 @@ export const api = {
     return st;
   },
 
+  // Public, works before login
+  async getLegalText(type: 'imprint' | 'privacy'): Promise<string> {
+    const res = await request<{ content: string }>(`/api/v1/legal/${type}`);
+    return res.content || '';
+  },
+
   async logout(): Promise<void> {
     try {
       await request<{ success: boolean }>('/api/v1/auth/logout', { method: 'POST' });