fix: vendor dashboard fonts + vault_exec docs cleanup

- Geist/Geist Mono via @fontsource inside the Vite bundle — the
  Google Fonts <link> was blocked by the nginx CSP (style-src 'self'),
  so the dashboard silently fell back and phoned home on self-hosted
  instances. Fonts now serve from /assets under 'self' — CSP-clean,
  airgapped, no console violation.
- mcp README: vault_exec described the old deny-list; rewritten for
  the allow-list contract.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/frontend/index.html

@@ -16,13 +16,6 @@
     <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent" />
     <meta name="apple-mobile-web-app-title" content="Primora" />
 
-    <link rel="preconnect" href="https://fonts.googleapis.com" />
-    <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
-    <link
-      href="https://fonts.googleapis.com/css2?family=Geist:wght@400;500;600;700&family=Geist+Mono:wght@400;500&display=swap"
-      rel="stylesheet"
-    />
-
     <style>
       html { background: #0a0a0b; color-scheme: dark; }
       html.light { background: #fafaf9; color-scheme: light; }