refactor(theme)!: remove deprecated `outlineTitle` option

BREAKING CHANGE: The deprecated `themeConfig.outlineTitle` option has been removed. Use `themeConfig.outline.label` instead.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/client/theme-default/composables/outline.ts

@@ -13,7 +13,6 @@ export function resolveTitle(theme: DefaultTheme.Config): string {
     (typeof theme.outline === 'object' &&
       !Array.isArray(theme.outline) &&
       theme.outline.label) ||
-    theme.outlineTitle ||
     'On this page'
   )
 }

types/default-theme.d.ts

@@ -30,13 +30,6 @@ export namespace DefaultTheme {
      */
     outline?: Outline | Outline['level'] | false
 
-    /**
-     * @deprecated Use `outline.label` instead.
-     *
-     * @default 'On this page'
-     */
-    outlineTitle?: string
-
     /**
      * The nav items.
      */