chore(web): port upstream preview and layout fixes (2026-10-09 round 3)

[TheDarkPhantom]

Upstream shipped a batch of file-preview and layout fixes on 2026-10-08. This hand-ports the ones that apply to the fork's diverged code and records why the rest do not.

Ported

• fix(web): images, video, HTML and PDF preview in a thread before its first message pingdotgg/t3code#17352 (Julius Marminge) Images, HTML and PDF now preview in a thread before its first message. A draft has no server thread, so the asset request resolved to "not found". Drafts now send their workspace root on the existing workspace-file resource (the fork's workspaceRoot override, honoured only for registered projects) instead of upstream's new draft-workspace-file resource, which depends on upstream's filesystem-read auth scope.
• fix(web): centered scrollers no longer shift when the scrollbar appears pingdotgg/t3code#17077 (maria) Centered scrollers no longer shift sideways when the scrollbar appears. Ports the require-centered-scroll-gutter lint rule and fixes the four fork sites it flags: preview empty state, preview unreachable page, usage page, and a reasoned suppression on the image preview, which never scrolls.
• fix(web): file previews handle downloads, in-page links, and repo paths, and favicons stop leaking internal hosts pingdotgg/t3code#16950 (Julius Marminge)
  • HTML previews and inline HTML attachments allow downloads (allow-downloads in the iframe sandbox and the asset CSP).
  • Markdown headings get GitHub-style ids, so table-of-contents links work. An in-page link scrolls to its heading without writing the fragment to the URL. Desktop routes through the hash, so the old behaviour navigated away from the thread.
  • Favicon requests send only a public hostname, never the port, and skip private, tailnet, reserved and internal-looking hosts. The fork had no filtering, so upstream's host classification comes over as a new apps/web/src/lib/faviconHost.ts, and chat link favicons now go through it.

Skipped

• fix(web): collapsed composer reserves room for wide send actions pingdotgg/t3code#17016 collapsed composer reserves room for wide send actions. The fork has no collapsed/resting composer (upstream fix(web): collapse the resting composer pingdotgg/t3code#7855 was never merged), so the overlap can't happen.
• fix(web): show a row spinner instead of a banner when expanding a folder pingdotgg/t3code#17378 row spinner when expanding a folder. The fork's file tree loads every entry in one request. Expanding a folder fetches nothing, so there is no banner to replace.
• fix(web): add room for thread timeline markers pingdotgg/t3code#17372 room for thread timeline markers. The fork has neither ChatCanvas nor timeline markers.
• The repo-path part of fix(web): file previews handle downloads, in-page links, and repo paths, and favicons stop leaking internal hosts pingdotgg/t3code#16950 (inline code in a rendered file resolving from the repo root). The fork already resolves inline code from the workspace root and has no sibling-relative base directory.

Verification

• vp test run FilePreviewPanel.test.ts, lib/favicon.test.ts, markdown-heading-ids.test.ts, markdown-links.test.ts, server http.test.ts: 94 passed
• vp lint on changed files: 0 errors. Linting apps and packages with the new rule turned up exactly the four sites fixed here.
• tsgo --noEmit for apps/web, apps/server, oxlint-plugin-t3code: clean
• vp run check:avicode and vp run test:avicode: pass

🤖 Generated with Claude Code