fix(security): audit batch B — SPA CSP/security headers + vite upgrade

Implements AUDIT_REPORT.md M-2 and M-5:
- M-2: CSP (script-src 'self'; frame-ancestors 'none'; object-src 'none'; ...),
  nosniff, X-Frame-Options DENY, Referrer-Policy on the SPA nginx, plus gzip for
  the bundle. The inline theme bootstrap moved to /theme-init.js so script-src
  'self' holds with no inline scripts.
- M-5: vite 5 -> 8 (+ plugin-react 6) — clears the dev-only esbuild advisories;
  npm audit now reports 0 vulnerabilities including dev deps. Build verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/public/theme-init.js

@@ -0,0 +1,9 @@
+// Applied before first paint to avoid a flash of the wrong theme. Lives in a file (not
+// inline) so the SPA can ship a CSP with script-src 'self' (AUDIT M-2).
+(function () {
+  try {
+    // Dark-first: default new users to dark unless they've chosen light.
+    var t = localStorage.getItem('ii:theme') || 'dark';
+    if (t === 'dark') document.documentElement.classList.add('dark');
+  } catch (e) {}
+})();