Set user's 24h preference from their current OS locale (#29651)

Fixes: https://github.com/go-gitea/gitea/issues/28371

Fixed by using a JS solution that formats according to `lang`, but alters the 24h format setting as per user's locale. This will work for all tooltips:

<img width="243" alt="Screenshot 2024-03-07 at 23 03 35" src="https://github.com/go-gitea/gitea/assets/115237/6d16c71c-6786-4eda-8cdc-50ec68ba62c6">
<img width="250" alt="Screenshot 2024-03-07 at 23 03 17" src="https://github.com/go-gitea/gitea/assets/115237/4e26bbb7-12df-4b81-bd37-14705e87e8f7">
<img width="310" alt="Screenshot 2024-03-07 at 23 14 34" src="https://github.com/go-gitea/gitea/assets/115237/1ef599f0-6401-4e19-b1da-59cdfc09b0f6">

I think there is only one other place in the UI where we render such absolute dates, which is in the actions view and which I've also fixed:

<img width="275" alt="Screenshot 2024-03-07 at 23 04 00" src="https://github.com/go-gitea/gitea/assets/115237/df0fbe1f-96ee-4338-ab5e-2b10e215005d">

3 files changed, 32 insertions, 3 deletions

diff --git a/web_src/js/components/RepoActionView.vue b/web_src/js/components/RepoActionView.vue
index 97cd05b45b..de9625b143 100644
--- a/web_src/js/components/RepoActionView.vue
+++ b/web_src/js/components/RepoActionView.vue
@@ -3,7 +3,7 @@ import {SvgIcon} from '../svg.js';
 import ActionRunStatus from './ActionRunStatus.vue';
 import {createApp} from 'vue';
 import {toggleElem} from '../utils/dom.js';
-import {getCurrentLocale} from '../utils.js';
+import {formatDatetime} from '../utils/time.js';
 import {renderAnsi} from '../render/ansi.js';
 import {POST, DELETE} from '../modules/fetch.js';
 
@@ -167,7 +167,7 @@ const sfc = {
       const logTimeStamp = document.createElement('span');
       logTimeStamp.className = 'log-time-stamp';
       const date = new Date(parseFloat(line.timestamp * 1000));
-      const timeStamp = date.toLocaleString(getCurrentLocale(), {timeZoneName: 'short'});
+      const timeStamp = formatDatetime(date);
       logTimeStamp.textContent = timeStamp;
       toggleElem(logTimeStamp, this.timeVisible['log-time-stamp']);
       // for "Show seconds"
diff --git a/web_src/js/modules/tippy.js b/web_src/js/modules/tippy.js
index 27f371fd88..489afc0ae1 100644
--- a/web_src/js/modules/tippy.js
+++ b/web_src/js/modules/tippy.js
@@ -1,5 +1,6 @@
 import tippy, {followCursor} from 'tippy.js';
 import {isDocumentFragmentOrElementNode} from '../utils/dom.js';
+import {formatDatetime} from '../utils/time.js';
 
 const visibleInstances = new Set();
 
@@ -93,8 +94,15 @@ function attachTooltip(target, content = null) {
 }
 
 function switchTitleToTooltip(target) {
-  const title = target.getAttribute('title');
+  let title = target.getAttribute('title');
   if (title) {
+    // apply custom formatting to relative-time's tooltips
+    if (target.tagName.toLowerCase() === 'relative-time') {
+      const datetime = target.getAttribute('datetime');
+      if (datetime) {
+        title = formatDatetime(new Date(datetime));
+      }
+    }
     target.setAttribute('data-tooltip-content', title);
     target.setAttribute('aria-label', title);
     // keep the attribute, in case there are some other "[title]" selectors
diff --git a/web_src/js/utils/time.js b/web_src/js/utils/time.js
index 3284e893e1..1848792c98 100644
--- a/web_src/js/utils/time.js
+++ b/web_src/js/utils/time.js
@@ -1,4 +1,5 @@
 import dayjs from 'dayjs';
+import {getCurrentLocale} from '../utils.js';
 
 // Returns an array of millisecond-timestamps of start-of-week days (Sundays)
 export function startDaysBetween(startDate, endDate) {
@@ -44,3 +45,23 @@ export function fillEmptyStartDaysWithZeroes(startDays, data) {
 
   return Object.values(result);
 }
+
+let dateFormat;
+
+// format a Date object to document's locale, but with 24h format from user's current locale because this
+// option is a personal preference of the user, not something that the document's locale should dictate.
+export function formatDatetime(date) {
+  if (!dateFormat) {
+    // TODO: replace `hour12` with `Intl.Locale.prototype.getHourCycles` once there is broad browser support
+    dateFormat = new Intl.DateTimeFormat(getCurrentLocale(), {
+      day: 'numeric',
+      month: 'short',
+      year: 'numeric',
+      hour: 'numeric',
+      hour12: !Number.isInteger(Number(new Intl.DateTimeFormat([], {hour: 'numeric'}).format())),
+      minute: '2-digit',
+      timeZoneName: 'short',
+    });
+  }
+  return dateFormat.format(date);
+}