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>

frontend/components.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "https://ui.shadcn.com/schema.json",
+  "style": "default",
+  "rsc": false,
+  "tsx": true,
+  "tailwind": {
+    "config": "tailwind.config.js",
+    "css": "src/index.css",
+    "baseColor": "slate",
+    "cssVariables": true,
+    "prefix": ""
+  },
+  "aliases": {
+    "components": "@/components",
+    "utils": "@/lib/utils",
+    "ui": "@/components/ui",
+    "lib": "@/lib",
+    "hooks": "@/hooks"
+  }
+}