Load mentionValues asynchronously (#36739)

Eliminate a few database queries on all issue and pull request pages by
moving mention autocomplete data to async JSON endpoints fetched
on-demand when the user types `@`.

See https://github.com/go-gitea/gitea/pull/36739#issuecomment-3963184858
for the full table of affected pages.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/globals.d.ts

@@ -29,7 +29,6 @@ interface Window {
     pageData: Record<string, any>,
     notificationSettings: Record<string, any>,
     enableTimeTracking: boolean,
-    mentionValues: Array<import('./types.ts').MentionValue>,
     mermaidMaxSourceCharacters: number,
     i18n: Record<string, string>,
   },