feat(frontend): add API client, layout, and routing

- Create Axios-based API client with sources, jobs, and dashboard endpoints
- Add responsive Layout component with navigation sidebar
- Set up React Router with Dashboard, Backups, and Settings routes
- Configure TanStack Query provider with default options
- Use lucide-react for navigation icons

frontend/node_modules/postcss/lib/declaration.js

@@ -0,0 +1,24 @@
+'use strict'
+
+let Node = require('./node')
+
+class Declaration extends Node {
+  get variable() {
+    return this.prop.startsWith('--') || this.prop[0] === '$'
+  }
+
+  constructor(defaults) {
+    if (
+      defaults &&
+      typeof defaults.value !== 'undefined' &&
+      typeof defaults.value !== 'string'
+    ) {
+      defaults = { ...defaults, value: String(defaults.value) }
+    }
+    super(defaults)
+    this.type = 'decl'
+  }
+}
+
+module.exports = Declaration
+Declaration.default = Declaration