Convert frontend code to typescript (#31559)

None of the frontend js/ts files was touched besides these two commands
(edit: no longer true, I touched one file in
61105d0618
because of a deprecation that was not showing before the rename).

`tsc` currently reports 778 errors, so I have disabled it in CI as
planned.

Everything appears to work fine.

web_src/js/components/ActivityHeatmap.vue

@@ -1,4 +1,4 @@
-<script>
+<script lang="ts">
 // TODO: Switch to upstream after https://github.com/razorness/vue3-calendar-heatmap/pull/34 is merged
 import {CalendarHeatmap} from '@silverwind/vue3-calendar-heatmap';