fix(app): expand Windows file tree folders

[opencode-agent]

Summary

• canonicalize Windows directory separators before keying file-tree expansion state
• cover trailing and nested backslash paths in the path helper unit suite
• add a headless Chromium regression that feeds the real V2 file tree Windows-shaped directory entries, expands a folder, and opens its nested file

Investigation

The server appends the platform separator to directory entries, so Windows returns paths such as frontend\\. The V2 row model canonicalizes this to frontend, while normalizeDir() previously preserved the backslash and stored expansion state under frontend\\. Rendering queried frontend, so the state never matched and aria-expanded remained false.

Confirmed reports:

• Closes V2 desktop: file tree folders don't expand on Windows (trailing path.sep breaks normalizeDir) #36734
• Closes The file resource panel in WebUI cannot expand folders. #37408
• Folder-expansion portion of [Bug]: Agents not visible when switching with Ctrl+. + folders not expanding in central file explorer (Desktop v1.18.1, Windows) #36979
• Same symptom in [BUG]please auto or supply a button to refresh the script viewer & file tree viewer #37209, plus mentions in Desktop: wtf is the new tab layout, tab titles dont fit anymore on screen #36936 and [FEATURE]: Vertical tabs #36942

Supersedes stale PR #36743 with a clean change based on current dev.

Checks

• bun test --preload ./happydom.ts ./src/context/file/path.test.ts — 38 passed
• bun typecheck — passed
• bun run test:e2e -- e2e/regression/open-file-expand-folder.spec.ts --reporter=line — passed
• bun run test:e2e -- e2e/regression/review-open-file.spec.ts e2e/regression/file-browser-sidebar-tab-switch.spec.ts --reporter=line — 2 passed

Requested by: @Hona (Hona via Slack)