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>

services/context/context.go

@@ -105,6 +105,7 @@ func NewTemplateContextForWeb(ctx reqctx.RequestContext, req *http.Request, loca
 	tmplCtx["Locale"] = locale
 	tmplCtx["AvatarUtils"] = templates.NewAvatarUtils(ctx)
 	tmplCtx["RenderUtils"] = templates.NewRenderUtils(ctx)
+	tmplCtx["MiscUtils"] = templates.NewMiscUtils(ctx)
 	tmplCtx["RootData"] = ctx.GetData()
 	tmplCtx["Consts"] = map[string]any{
 		"RepoUnitTypeCode":            unit.TypeCode,