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/vitest.setup.ts

@@ -10,15 +10,6 @@ window.config = {
   pageData: {},
   notificationSettings: {},
   enableTimeTracking: true,
-  mentionValues: [
-    {key: 'user1 User 1', value: 'user1', name: 'user1', fullname: 'User 1', avatar: 'https://avatar1.com'},
-    {key: 'user2 User 2', value: 'user2', name: 'user2', fullname: 'User 2', avatar: 'https://avatar2.com'},
-    {key: 'org3 User 3', value: 'org3', name: 'org3', fullname: 'User 3', avatar: 'https://avatar3.com'},
-    {key: 'user4 User 4', value: 'user4', name: 'user4', fullname: 'User 4', avatar: 'https://avatar4.com'},
-    {key: 'user5 User 5', value: 'user5', name: 'user5', fullname: 'User 5', avatar: 'https://avatar5.com'},
-    {key: 'org6 User 6', value: 'org6', name: 'org6', fullname: 'User 6', avatar: 'https://avatar6.com'},
-    {key: 'org7 User 7', value: 'org7', name: 'org7', fullname: 'User 7', avatar: 'https://avatar7.com'},
-  ],
   mermaidMaxSourceCharacters: 5000,
   i18n: {},
 };