feat(web): preview Markdown and HTML by default

[yaacovcorcos]

What changed

• Render Markdown and MDX by default when no saved user preference exists.
• Open HTML and HTM files from the file tree in the existing integrated browser.
• Preserve the remembered Markdown source/preview choice and source view for line-targeted links.
• Keep HTML source available through the file-tree context menu.
• Fall back to source if browser preview is unsupported or fails.
• Document the resulting file-preview behavior.

Why

Scient users usually open Markdown and generated HTML to read the result. Requiring an extra preview action on every file made the common path slower while source access can remain explicit and reliable.

Upstream boundary

The product policy and preview orchestration live under apps/web/src/scient/fileOpening. The inherited T3 file tree, right-panel store, editor, asset serving, and integrated-browser engine remain intact. Three inherited UI files receive only narrow integration calls.

Verification

• pnpm exec vp fmt --check
• focused vp lint --report-unused-disable-directives
• pnpm --filter @t3tools/web typecheck
• pnpm --filter @t3tools/web test — 2,104 tests passed
• pnpm --filter @t3tools/web build
• git diff --check

No browser automation or visual test was performed, as requested.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	11.3 KiB	11.3 KiB	+9 B (+0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	5.5 KiB	5.5 KiB	−11 B (−0.2%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	5.8 KiB	5.9 KiB	+20 B (+0.3%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	49.7 KiB	49.7 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	16	16	0 (0.0%)	21	✅
Claude	Total thread wire	11.3 KiB	11.3 KiB	−22 B (−0.2%)	15.1 KiB	✅
Claude	Thread snapshot wire	5.5 KiB	5.5 KiB	−3 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	5.9 KiB	5.8 KiB	−19 B (−0.3%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	50.6 KiB	50.6 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	16	16	0 (0.0%)	21	✅

Baseline: aff3f8a · PR result: 5b4966e · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 94.7 KiB
• Claude decoded thread snapshot: 95.4 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.