Fix failing theme editor tests and expose applyTheme API

- Added applyTheme function to JavaScript editor API in runtime.rs
- Fixed test_color_prompt_shows_suggestions to properly navigate to
  color field before pressing Enter
- Fixed test_theme_applied_immediately_after_save to complete the
  prompt flow (type theme name and confirm)

Author: claude

src/services/plugins/runtime.rs

@@ -2803,6 +2803,11 @@ impl TypeScriptRuntime {
                         core.ops.op_fresh_debug(message);
                     },
 
+                    // Theme operations
+                    applyTheme(themeName) {
+                        return core.ops.op_fresh_apply_theme(themeName);
+                    },
+
                     // Clipboard
                     copyToClipboard(text) {
                         core.ops.op_fresh_set_clipboard(text);