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.

src/ui/paths.cpp

@@ -147,6 +147,11 @@ std::filesystem::path GetSessionStatePath() {
     return user_data.empty() ? std::filesystem::path{} : user_data / "session_state.json";
 }
 
+std::filesystem::path GetFirstRunStatePath() {
+    auto user_data = GetUserDataDirectory();
+    return user_data.empty() ? std::filesystem::path{} : user_data / "first_run_state.json";
+}
+
 std::filesystem::path GetUiPagePath(const std::string& page_name) {
     const auto exe_dir = GetExecutableDirectory();
     if (exe_dir.empty()) {
@@ -182,6 +187,10 @@ std::string GetHomeUrl() {
     return InternalUrlForSlug("home");
 }
 
+std::string GetSetupUrl() {
+    return InternalUrlForSlug("setup");
+}
+
 std::string GetSettingsUrl() {
     return InternalUrlForSlug("settings");
 }