feat: HTML viewer

[jamietre]

Summary

• .html / .htm files show a Rendered toolbar button that displays the file in an inline iframe
• New GET /api/v1/raw/{source}/{*path} endpoint serves files with source/path in the URL path rather than query params — this lets the browser resolve relative assets (images, CSS) correctly as sibling requests on the same endpoint
• htmlFormat preference persisted to user profile (same pattern as markdownFormat / rtfFormat)

Test plan

• Open an HTML file — toolbar shows "Rendered" button
• Click Rendered — file displays in an iframe
• Images and CSS from relative paths load correctly
• Click Plain — returns to extracted text view
• Preference persists across page reloads
• Non-HTML files show no Rendered button

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