fix(security): safe links and security headers (FLAW-28, FLAW-29)

safeHref lets only http, https and mailto become links (Markdown viewer,
outliner, legal links); javascript: and friends show as text. A
middleware sets a Content-Security-Policy (scripts only from MindSift,
no framing, connections only to this server), X-Frame-Options,
nosniff, Referrer-Policy and a Permissions-Policy. The two inline
scripts of index.html moved to files so script-src stays 'self'.

The second inline script also deleted every cache on each page load,
which undermined the offline start; it is gone.

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

frontend/src/components/LegalLinks.tsx

@@ -3,6 +3,7 @@ import { AuthStatus } from '../lib/types';
 import { api } from '../lib/api';
 import { MarkdownViewer } from '../lib/markdown';
 import { ReadingModal } from './ReadingModal';
+import { safeHref } from '../lib/safeUrl';
 
 type LegalType = 'imprint' | 'privacy';
 const TITLES: Record<LegalType, string> = { imprint: 'Impressum', privacy: 'Datenschutz' };
@@ -41,8 +42,8 @@ export const LegalLinks: React.FC<LegalLinksProps> = ({ legal, className = '', c
   return (
     <div className={`flex flex-wrap items-center justify-center gap-x-4 gap-y-1 text-[11px] text-muted ${className}`}>
       {entries.map((e) =>
-        e.url ? (
-          <a key={e.type} href={e.url} target="_blank" rel="noopener noreferrer" className="hover:text-main underline-offset-2 hover:underline">
+        e.url && safeHref(e.url) ? (
+          <a key={e.type} href={safeHref(e.url) as string} target="_blank" rel="noopener noreferrer" className="hover:text-main underline-offset-2 hover:underline">
             {TITLES[e.type]}
           </a>
         ) : (