UI: Git dirty preflight + terminal fix

[gabelul]

• Blocks parallel start when the project git working tree has non-ignorable uncommitted changes; surfaces a Git Dirty modal with one-click "Stash changes".
• Adds /api/projects//git/status + /git/stash endpoints and shows a "Git dirty" badge in Progress.
• Gatekeeper now ignores known Playwright artifacts and includes dirty-file details in rejection excerpts.
• Fixes Windows terminal spawning when pwsh path contains spaces.

How to verify:

• Run autocoder-ui, open a project with git changes, click Run in parallel mode → see Git Dirty modal.
• Click "Stash changes" → retry start should work.
• Open Term tab → terminal should launch even with pwsh in Program Files.

Tests:

• pytest -q
• npm -C ui run build