feat(html): render local HTML files with sandboxed assets

[wfnuser]

Summary

• Recognize .html/.htm files in local Space trees and preserve their extensions in routes, without changing Markdown behavior.
• Render original HTML in a read-only preview with a Source toggle. Users can copy HTML plus assets into a Space; no Markdown conversion or import wizard is required.
• Package relative CSS, JavaScript, images, fonts and supported media in memory, reusing the existing iframe sandbox/CSP.

Security and scope

• Local desktop only; Cloud native HTML document/resource reading is not implemented by this PR.
• Resource reads are restricted to the HTML document directory and descendants, with an extension allowlist; hidden paths, traversal and symbolic links are rejected.
• Limits: 8 MiB per file, 24 MiB aggregate resource bytes, 160 resource paths.
• Sandbox permits scripts without same-origin privileges; CSP blocks network requests, forms and nested remote content.
• No arbitrary filesystem access or write API is exposed to document scripts.
• No CPU/memory execution quota: hostile or buggy JavaScript can still cause performance problems.
• CSS @import, dynamic module/resource loading, and cross-document link navigation are not supported. HTML editing/comments/history controls are intentionally not exposed.

Validation

• npm test — rerun successfully before opening this PR.
• npm run lint and npm run build.
• Chromium and WebKit browser suite: 12 passing tests.
• Rust library suite: 114 passing tests; cargo fmt --check.
• Debug macOS application bundle built and launched.
• Real FLAIR Super Reading HTML + assets copied into a local demo Space: formulas, images, zoom dialog, anchor scrolling and training/inference tab interaction verified in the native client.

The real paper/demo assets are not included in this PR; committed fixtures are small synthetic regression cases.

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