fix(spx-gui): fix stage defaulting to widget tab when switching to stage

[xgopilot]

Requested by @nighca

Fixes #2960

Summary

• Removed the watcher in StageEditorState that automatically switched to the widgets tab whenever a widget was selected
• WidgetsEditorState initializes with the first widget pre-selected, which caused the watcher to trigger and switch to the widgets tab as soon as the stage was activated
• Now the stage correctly defaults to the code tab when first activated, matching expected behavior

Root Cause

In StageEditor.vue, StageEditorState had a watcher on widgetsState.selected:

watch(
  () => this.widgetsState.selected,
  (selectedWidget) => {
    if (selectedWidget != null) {
      this.select('widgets')  // This was the culprit
    }
  }
)

WidgetsEditorState initializes selectedIdRef with the first widget's ID from getStage().widgets[0]?.id. When the project loads and widgets become available, this triggers the watcher chain:

1. WidgetsEditorState auto-selects the first widget
2. The watcher in StageEditorState sees a non-null widget and switches to 'widgets' tab
3. Result: switching to stage always shows the widget tab instead of code tab

Fix

Removed the problematic watcher. The widgets tab is now only shown when:

• The user explicitly clicks the "Widgets" quick button (calls select('widgets'))
• The user explicitly selects a widget (calls selectWidget())
• State is restored from the URL route via selectByRoute()

This preserves the "restore prior selection" behavior described in the issue while fixing the incorrect default.

[xgopilot]

Clean, well-scoped fix. The removed watcher was the sole cause of the bug — all intentional widget-selection paths (selectWidget, selectByRoute, and click handlers) already call this.select('widgets') explicitly, so the watcher was both redundant and harmful for the initialization case. Unused watch import correctly cleaned up too. No issues found.