Set up comprehensive frontend testing infrastructure

- Install Jest for unit testing with React Testing Library
- Install Playwright for end-to-end testing
- Configure Jest with proper TypeScript support and module mapping
- Create test setup files and utilities for both unit and e2e tests

Components:
* Jest configuration with coverage thresholds
* Playwright configuration with browser automation
* Unit tests for LoginForm, AuthContext, and useSocketIO hook
* E2E tests for authentication, dashboard, and agents workflows
* GitHub Actions workflow for automated testing
* Mock data and API utilities for consistent testing
* Test documentation with best practices

Testing features:
- Unit tests with 70% coverage threshold
- E2E tests with API mocking and user journey testing
- CI/CD integration for automated test runs
- Cross-browser testing support with Playwright
- Authentication system testing end-to-end

🚀 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/node_modules/cssstyle/lib/properties/background.js

@@ -0,0 +1,52 @@
+"use strict";
+// FIXME:
+// * support multiple backgrounds
+// * also fix longhands
+
+const parsers = require("../parsers");
+const strings = require("../utils/strings");
+const backgroundImage = require("./backgroundImage");
+const backgroundPosition = require("./backgroundPosition");
+const backgroundRepeat = require("./backgroundRepeat");
+const backgroundAttachment = require("./backgroundAttachment");
+const backgroundColor = require("./backgroundColor");
+
+const shorthandFor = new Map([
+  ["background-image", backgroundImage],
+  ["background-position", backgroundPosition],
+  ["background-repeat", backgroundRepeat],
+  ["background-attachment", backgroundAttachment],
+  ["background-color", backgroundColor]
+]);
+
+module.exports.definition = {
+  set(v) {
+    v = parsers.prepareValue(v, this._global);
+    if (/^none$/i.test(v)) {
+      for (const [key] of shorthandFor) {
+        this._setProperty(key, "");
+      }
+      this._setProperty("background", strings.asciiLowercase(v));
+    } else if (parsers.hasVarFunc(v)) {
+      for (const [key] of shorthandFor) {
+        this._setProperty(key, "");
+      }
+      this._setProperty("background", v);
+    } else {
+      this._shorthandSetter("background", v, shorthandFor);
+    }
+  },
+  get() {
+    let val = this.getPropertyValue("background");
+    if (parsers.hasVarFunc(val)) {
+      return val;
+    }
+    val = this._shorthandGetter("background", shorthandFor);
+    if (parsers.hasVarFunc(val)) {
+      return "";
+    }
+    return val;
+  },
+  enumerable: true,
+  configurable: true
+};