Improve file and diff inspection

[powerfooI]

Summary

• share one Highlight.js language-resolution and token-coloring pipeline between File Preview and Diff Viewer
• automatically collapse generated, truncated, and 1,000+ changed-line diffs while keeping manual expansion available and skipping collapsed rendering/highlighting work
• detect .gitattributes linguist-generated markers in diff summaries and prefer an ordinary file for the initial Diff selection

Verification

• bun run format:check
• bun run lint
• cd web && bun run typecheck
• cd server && bun run typecheck
• cd web && bun run build
• TMPDIR=/tmp bun test server/src/workspace/git-diff.test.ts web/src/components/diffAutoCollapse.test.ts web/src/components/syntaxHighlighting.test.ts (12 pass)
• full suite excluding server/src/worktree/remove.test.ts (576 pass, 1 skip)

Known test environment issue

TMPDIR=/tmp bun test server/src/worktree/remove.test.ts has one unrelated failure: stops a stable process whose cwd is inside the checkout times out while cleaning up its spawned process. The test file is unchanged from main; its other 20 tests pass.

Screenshots

Not included: these changes affect syntax rendering and automatic behavior inside workspace-backed Files/Diff views, and no Herdr workspace fixture is available in the submission environment.