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/lib/__tests__/safeUrl.test.tsx

@@ -0,0 +1,38 @@
+import { describe, it, expect } from 'vitest';
+import { safeHref } from '../safeUrl';
+
+describe('safeHref', () => {
+  it('keeps web and mail links', () => {
+    expect(safeHref('https://example.com/a?b=1')).toBe('https://example.com/a?b=1');
+    expect(safeHref(' http://example.com ')).toBe('http://example.com/');
+    expect(safeHref('mailto:anna@example.com')).toBe('mailto:anna@example.com');
+  });
+
+  it('drops everything that could run code or load local data', () => {
+    for (const bad of [
+      'javascript:alert(1)',
+      'JaVaScRiPt:alert(1)',
+      ' javascript:alert(1)',
+      'java\tscript:alert(1)',
+      'java\nscript:alert(1)',
+      'data:text/html,<script>alert(1)</script>',
+      'vbscript:msgbox(1)',
+      'file:///etc/passwd',
+      '/relative/path',
+      '',
+    ]) {
+      expect(safeHref(bad)).toBeNull();
+    }
+  });
+});
+
+describe('Markdown links in task notes', () => {
+  it('render javascript: links as text, web links as links', async () => {
+    const { render } = await import('@testing-library/react');
+    const { MarkdownViewer } = await import('../markdown');
+    const { container } = render(<MarkdownViewer content={'[Rechnung](javascript:alert(1)) und [Seite](https://example.com)'} />);
+    const links = Array.from(container.querySelectorAll('a'));
+    expect(links.map((a) => a.getAttribute('href'))).toEqual(['https://example.com/']);
+    expect(container.textContent).toContain('Rechnung');
+  });
+});