feat: render html in quick preview with local assets

[beihai23]

what

Quick preview now renders .html / .htm files instead of showing raw source:

• new html preview kind in src/lib/preview.ts (moved out of the text bucket)
• rendered in a sandboxed iframe (sandbox="allow-scripts" — scripts run in an opaque origin, no same-origin/local-file access)
• htmlDocWithBase() injects a <base> pointing at the file's directory via convertFileSrc(), so relative images/css/scripts resolve
• asset protocol enabled (scope: ["**"], same surface as the existing fs:allow-read-file ** permission) + protocol-asset cargo feature
• white canvas background (pages assume white, independent of app theme)
• "open as text" button stays available for viewing source

security notes

• iframe is sandboxed: opaque origin, no top navigation, no popups
• asset protocol scope matches what the fs plugin can already read, so no new privilege

verification

• bun test — 73 pass (incl. new htmlDocWithBase cases)
• tsc --noEmit clean
• cargo check clean