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>

web_src/js/features/repo-issue-pull.ts

@@ -1,5 +1,4 @@
 import {createApp} from 'vue';
-import PullRequestMergeForm from '../components/PullRequestMergeForm.vue';
 import {GET, POST} from '../modules/fetch.ts';
 import {fomanticQuery} from '../modules/fomantic/base.ts';
 import {createElementFromHTML} from '../utils/dom.ts';
@@ -63,10 +62,11 @@ function initRepoPullRequestCommitStatus(el: HTMLElement) {
   }
 }
 
-function initRepoPullRequestMergeForm(box: HTMLElement) {
+async function initRepoPullRequestMergeForm(box: HTMLElement) {
   const el = box.querySelector('#pull-request-merge-form');
   if (!el) return;
 
+  const {default: PullRequestMergeForm} = await import(/* webpackChunkName: "PullRequestMergeForm" */ '../components/PullRequestMergeForm.vue');
   const view = createApp(PullRequestMergeForm);
   view.mount(el);
 }