fix(git): fix 17 missing async/await bugs + UX overhaul

Backend:
- fix 17 missing 'await' in git route handlers causing silent no-ops
  (branches, diff, history, commit, push, pull, merge, promote, sync)
- fix async coroutine passed to run_blocking in git_plugin.py

Frontend:
- add collapsible 'How it works' onboarding (GitHelpPanel)
- add status legend with color-coded repository statuses
- i18n: add 50+ missing keys, replace hardcoded strings
- add Refresh button in modal header
- add PROD deploy confirmation dialog (replaces browser prompt())
- add CommitHistory to workspace tab with timeline nodes
- add post-commit success banner with next-step guidance
- increase success toast duration to 8s
- group local/remote branches in selector (optgroup)
- format last_modified dates timezone-aware
- change PROD badge from red to neutral indigo
- extract shared resolveGitStatusToken to git-utils.ts
- fix 'slug' label regression
- remove dead init_repo_button key

UI/UX audit fixes:
- add descriptions to Create/Init buttons in init panel
- add actionable CTA to server mismatch warning
- improve checkbox text phrasing

backend/.env.example

@@ -1,7 +1,8 @@
 # ── Required: Secrets ──
 # Generate with: python -c "import secrets; print(secrets.token_urlsafe(32))"
 AUTH_SECRET_KEY=change-me-to-a-random-secret
-ENCRYPTION_KEY=change-me-to-a-fernet-key
+# Generate with: python3 -c "import base64,os; print(base64.urlsafe_b64encode(os.urandom(32)).decode())"
+ENCRYPTION_KEY=
 
 # ── Required: Database ──
 # PostgreSQL via docker-compose: