refactor(sandbox): remove preview-fetch proxy

[guitavano]

Summary

• Remove the preview-fetch CORS proxy route from sandbox-proxy — useLiveMeta and useDecofile now fetch /live/_meta and /.decofile directly from the preview URL
• Delete preview-fetch-url.ts (URL builder with proxy fallback) and its tests
• Thread previewUrl through all call sites (preview.tsx, sections-editor.tsx, content-browser.tsx, use-main-panel-tabs.ts)
• Add cache: "no-store" to bypass browser HTTP caching on direct fetches
• Reduce file-changed debounce from 2s to 1s

Test plan

• Save a file in the sandbox editor and verify /live/_meta reflects the change within ~1s
• Verify sections editor, content browser, and preview all load metadata correctly
• Confirm no regressions in the CMS visual editor flow

🤖 Generated with Claude Code

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Summary by cubic

Removed the preview-fetch CORS proxy and now fetch /.decofile and /live/_meta directly from the sandbox preview URL for simpler, faster metadata loads. Also added cache: "no-store" and reduced file-change debounce to 1s for quicker updates.

• Refactors
  • Removed proxy route from sandbox-proxy.ts.
  • Deleted preview-fetch-url.ts and its tests.
  • Switched useLiveMeta and useDecofile to direct fetches with cache: "no-store".
  • Threaded previewUrl through preview.tsx, sections-editor.tsx, content-browser.tsx, and use-main-panel-tabs.ts.
  • Shortened live meta debounce from 2s to 1s in sandbox-events-context.tsx.

^{Written for commit c510503. Summary will update on new commits.}