feat(app): render markdown files in file view

[opencode-agent]

Summary

• render manually opened .md files with the existing Markdown renderer
• add a file toolbar showing the project-relative path
• let Markdown files switch between rendered and source views
• keep non-Markdown files in the source viewer
• move the existing Open in control into the file toolbar for open files and target the active file
• add regression coverage for the toolbar and Markdown view toggle

Checks

• bun typecheck (packages/app)
• bun run check with Node 24
• production app build
• focused review-open-file.spec.ts Playwright regression
• focused Open in app/path unit tests
• latest PR CI passes on Linux and Windows

Requested by: @Hona (Hona via Slack)