fix(nav): prevent workspace card diff overlap

[ovdmar]

Summary

• Reserve the full right-control lane on workspace cards so PR diff totals do not render under the structured workspace + / collapse controls.
• Add a Vitest regression guard for the CSS lane sizing contract.

Plan

Inline / trivial

Test plan

• make check passes locally
• pnpm e2e passes locally
• pnpm smoke: not run; daemon HTTP surface unchanged

How to QA

1. Pull the branch: git checkout fb-nav-pr-diff-overlap
2. Install: pnpm install
3. Full check: make check
4. Start the daemon + web in dev mode: pnpm dev
   • Daemon: http://localhost:4010 (or the worktree's resolved port)
   • Web cockpit: http://localhost:5173
5. Open a structured workspace row that shows PR diff totals and the add-worktree + control; verify the + remains clear of the +added/-deleted totals.
6. Hover/focus the workspace row and verify the right-side controls remain clickable without covering the diff totals.

For E2E coverage:

• pnpm e2e to run the full suite