Add first-run setup and theme synchronization

Introduce first-run setup flow and live chrome theme syncing.

- Add first_run_state.cpp/.h to read/write a first_run_state.json under user data and decide whether to show the setup UI.
- Wire first-run logic into NebulaController: track first_run_setup_active_, create initial setup tab, defer/bring up chrome browser accordingly, and add CompleteFirstRunSetup() to persist state and finish setup.
- Add SendThemeToChromeSurfaces() and handle "theme-update" and "complete-first-run" chrome commands; restrict setup completion to setup frame.
- Expose GetFirstRunStatePath() and GetSetupUrl() in UI path helpers and include the state file in the build list (CMakeLists.txt).
- Update chrome UI: new CSS variables and styles for tabs/url-bar; chrome.js can apply themes (applyTheme), persist/load theme, and listen for storage updates to apply theme changes live.
- Update customization.js, settings.js, and setup.js to normalize/persist themes, send theme updates to the native host (or fallback), and communicate completion via the native bridge when available; include customization.js in setup.html.

These changes allow the app to run an interactive first-run setup and keep the separate chrome UI in sync with user-selected themes.

ui/js/customization.js

@@ -4,7 +4,7 @@
  */
 
 class BrowserCustomizer {
-  constructor() {
+  constructor(options = {}) {
     this.defaultTheme = {
       name: 'Default',
       colors: {
@@ -286,6 +286,10 @@ class BrowserCustomizer {
       }
     };
 
+    if (options.skipInit) {
+      return;
+    }
+
     this.currentTheme = this.loadTheme();
     this.activeThemeName = this.loadActiveThemeName();
     this.init();
@@ -584,9 +588,13 @@ class BrowserCustomizer {
     // This will be called to apply theme to home.html and other pages
     this.saveTheme();
 
-    // Send theme update to host (for settings webview)
-    if (window.electronAPI && typeof window.electronAPI.sendToHost === 'function') {
-      window.electronAPI.sendToHost('theme-update', this.currentTheme);
+    const themePayload = JSON.stringify(this.currentTheme);
+
+    // Send theme update to host so the separate chrome browser can update live.
+    if (window.nebulaNative && typeof window.nebulaNative.postMessage === 'function') {
+      window.nebulaNative.postMessage('theme-update', themePayload);
+    } else if (window.electronAPI && typeof window.electronAPI.sendToHost === 'function') {
+      window.electronAPI.sendToHost('theme-update', themePayload);
     }
     // Fallback: send via postMessage (for iframe embedding)
     try {