Respect site cfg_json theme on first-run; add user_theme_selected flag; set flag when user selects theme or URL param

src/lib/stores/ae_stores.ts

@@ -68,6 +68,9 @@ const ae_app_local_data_defaults: key_val = {
     theme: 'light',
     theme_mode: 'light',
     theme_name: 'AE_Firefly', // OSIT default theme
+    // When true the user has explicitly chosen a theme (dropdown or URL param).
+    // Use this to avoid overwriting an explicit user choice with a site default.
+    user_theme_selected: false,
     font_size_mode: 'default', // 'default' | 'larger' | 'smaller'
     iframe: false,