feat: polish UI demo workflow

[AusafMo]

Summary

• polish the desktop UI toolbar, branch workflow controls, diff containment, and overflow behavior
• remove the misleading live-drift restore action while keeping restore on historical versions
• add synthetic support-control-plane demo data, drift seeding, and README setup instructions

Verification

• pytest -q
• git diff --check
• browser-tested localhost UI on disposable Mongo DB cfgit_ui_polish_20260628 at 1440px and 1280px desktop widths
• exercised drift diff, impact, adopt, historical restore, search, theme toggle, branch create, draft commit, branch diff, PR open, and PR merge