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/index.html

@@ -40,14 +40,7 @@
 
     <link rel="manifest" href="/manifest.json" />
     <title>MindSift</title>
-    <script>
-      window.__deferredPwaPrompt = null;
-      window.addEventListener('beforeinstallprompt', function (e) {
-        e.preventDefault();
-        window.__deferredPwaPrompt = e;
-        window.dispatchEvent(new CustomEvent('mindsift:pwa-ready'));
-      });
-    </script>
+    <script src="/pwa-install.js"></script>
   </head>
   <body class="bg-[#0b0f19] text-[#e3e3e3] antialiased selection:bg-[#8ab4f8]/30 selection:text-[#8ab4f8]">
     <!-- Shown until React mounts and replaces it: full blue with the logo, no text, no white flash -->
@@ -58,15 +51,5 @@
       </div>
     </div>
     <script type="module" src="/src/main.tsx"></script>
-    <script>
-      if ('caches' in window) {
-        caches.keys().then((keys) => keys.forEach((k) => caches.delete(k))).catch(() => {});
-      }
-      if ('serviceWorker' in navigator) {
-        window.addEventListener('load', () => {
-          navigator.serviceWorker.register('/sw.js').catch(() => {});
-        });
-      }
-    </script>
   </body>
 </html>