feat: align tab bar with terminal window under workspace hierarchy & add automated test suite

- Restructure UI layout: tab bar and stage actions are now housed inside .workspace-stage (.workspace-header), aligning 100% with the terminal window (#main-viewport) below it
- Establish clear workspace hierarchy: Global Titlebar -> Left Workspace Sidebar -> Active Workspace Content Stage (Tab Bar + Terminal Viewport)
- Add active workspace indicator badge to titlebar header
- Introduce comprehensive automated test suite (test/test-runner.cjs) covering PTY process lifecycle, workspace hierarchy, stage alignment, tab creation, split panes, shortcuts, and themes
- Add 'npm test' script to package.json

package.json

@@ -5,6 +5,7 @@
   "main": "src/main/index.cjs",
   "scripts": {
     "start": "electron .",
+    "test": "electron test/test-runner.cjs",
     "rebuild": "electron-rebuild -f -w node-pty",
     "package:deb": "electron-builder --linux deb",
     "package:all": "electron-builder --linux deb AppImage",