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.h

@@ -9,10 +9,12 @@ std::filesystem::path GetExecutableDirectory();
 std::filesystem::path GetUserDataDirectory();
 std::filesystem::path GetCacheDirectory();
 std::filesystem::path GetSessionStatePath();
+std::filesystem::path GetFirstRunStatePath();
 std::filesystem::path GetUiPagePath(const std::string& page_name);
 std::string FilePathToUrl(std::filesystem::path path);
 std::string GetChromeUrl();
 std::string GetHomeUrl();
+std::string GetSetupUrl();
 std::string GetSettingsUrl();
 std::string GetDownloadsUrl();
 std::string GetBigPictureUrl();