Lazy-load some Vue components, fix heatmap chunk loading on every page (#36719)

Lazy-load 3 Vue components that are safe to defer (no pop-in effects).
This reduces `index-domready` from 515 KiB to 502 KiB (-2.5%).

The old `vue3-calendar-heatmap` vendor chunk (264 KiB) that previously
loaded on every page is eliminated entirely — it was mostly duplicate
`tippy.js` and `vue` copies that webpack had split out. The actual
heatmap library is only ~12 KiB minified, now inlined into the
`ActivityHeatmap` async chunk.

---------

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

templates/repo/issue/view_content/pull_merge_box.tmpl

@@ -302,7 +302,8 @@
 						</script>
 
 						{{$showGeneralMergeForm = true}}
-						<div id="pull-request-merge-form"></div>
+						{{/* The merge form is a Vue component. After mounted, it has a button for choosing merge style, so make it have min-height to avoid layout shifting */}}
+						<div id="pull-request-merge-form" class="tw-min-h-[40px]"></div>
 					{{else}}
 						{{/* no merge style was set in repo setting: not or ($prUnit.PullRequestsConfig.AllowMerge ...) */}}
 						<div class="divider"></div>