fix: proper CSP handling in basic-host sandboxing (HTTP headers, worker-src, document.write) (#234)

* fix: Move CSP to HTTP headers + add worker-src, frameDomains, baseUriDomains, permissions

Security improvements:
- CSP is now set via HTTP headers in serve.ts instead of meta tags
  (meta tag CSP can be tampered with by same-origin content)
- CSP passed as query param to sandbox.html for header-based enforcement

New CSP/permissions features (borrowed from PR #158):
- frameDomains: control frame-src directive for nested iframes
- baseUriDomains: control base-uri directive
- permissions: camera, microphone, geolocation via iframe allow attribute

WebGL fix:
- Use document.write() instead of srcdoc for inner iframe content
  (srcdoc creates opaque origin that breaks WebGL canvas updates)
- Add worker-src directive with blob: support (critical for WebGL apps
  like CesiumJS/Three.js that use workers for tile decoding, terrain
  processing, image processing)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* refactor: Remove permissions support (defer to PR #158)

Keep this PR focused on CSP security fixes only.
Permissions (camera, microphone, geolocation) will be
handled in #158.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* cleanup McpUiResourceCsp type usage / duplicate defs

* fix: guard against CSP injection in domain parameters

Validate CSP domain entries to reject characters that could:
- Break out of CSP directives (semicolons, newlines)
- Inject CSP keywords like 'unsafe-eval' (quotes)
- Inject multiple sources in one entry (spaces)

This prevents injection attacks where malicious domains could
override the security policy.

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

examples/basic-host/sandbox.html

@@ -3,8 +3,8 @@
   <head>
     <meta charset="utf-8" />
     <meta name="color-scheme" content="light dark">
-    <!-- CSP is set by serve.ts HTTP header - no meta tag needed here
-         The inner iframe's CSP is dynamically injected based on resource metadata -->
+    <!-- CSP is set via HTTP headers by serve.ts (based on ?csp= query param).
+         The inner iframe inherits this CSP since we use document.write(). -->
     <title>MCP-UI Proxy</title>
     <style>
       html,