Mount Changes only when visible

[selfcontained]

Summary

• mount the Changes pane only when it is the active single-pane tab or appears in either split pane
• keep the terminal mounted as before
• retain the existing non-virtualized Changes renderer and review-link behavior

Validation

• pnpm run check
• pnpm run finalize:web
• pnpm run test:e2e (178 passed, 12 skipped)
• Browser verification: Changes has 0 mounted diff views while Terminal is active and mounts all fixture files after selecting Changes
• Frontend UX review approved with no findings