Migrate frontend to shadcn/ui + add Gitea branch panel in endpoint editor

shadcn/ui migration:
- Installed shadcn components: Button, Input, Label, Card, Badge, Checkbox,
  Switch, Tabs, Dialog, Select, Tooltip, Toaster (sonner)
- Migrated all pages: Login, Dashboard, Settings, Endpoints, EndpointEditor,
  Folders, ApiKeys, Logs
- Replaced react-hot-toast with sonner
- Added CSS variables for theming
- Updated tailwind.config.js for shadcn

Gitea branch panel in EndpointEditor:
- GiteaBranchPanel component: branch selector, save to branch, compare
  with main, create PR, merge, commit history
- New backend endpoints: getEndpointFileContent, commitEndpoint,
  getEndpointCommits, getEndpointFromBranch, commitEndpointToBranch
- Frontend giteaApi: extended with endpoint-specific methods

Fixes:
- Fixed sync-all PostgreSQL-compatible UPDATE LIMIT
- .gitignore: added tmp/

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -12,4 +12,6 @@ coverage/
 *.sln
 *.sw?
 .claude
-.git_backup/
+.git_backup/
+tmp/
+!tmp/.gitkeep