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>

templates/repo/issue/fields/textarea.tmpl

@@ -9,8 +9,7 @@
 		{{template "shared/combomarkdowneditor" (dict
 			"CustomInit" true
 			"ContainerClasses" "tw-hidden"
-			"MarkdownPreviewInRepo" $.root.Repository
-			"MarkdownPreviewMode" "comment"
+			"MarkdownEditorContext" (ctx.MiscUtils.MarkdownEditorComment ctx.RootData.Repository)
 			"TextareaContent" .item.Attributes.value
 			"TextareaPlaceholder" .item.Attributes.placeholder
 			"DropzoneParentContainer" ".combo-editor-dropzone"