ui: home carousel height lock + bubble polish

- Home: lock category carousel height to the tallest page so the
  Official Recommendations section below does not jump up when
  swiping to a page with fewer categories.
- CollapsibleText: raise default threshold to 25 lines and tighten
  the spacing between the expand-all button and the timestamp
  (drop the fixed h-8 and use mt-1 instead of mt-1.5).
- formatTime: always render dates as yyyy/m/d HH:mm regardless of
  locale, matching the requested timestamp format.

src/components/messageStream/utils/formatTime.ts

@@ -1,34 +1,17 @@
-function localeFor(lang: string): string {
-  const locales: Record<string, string> = {
-    zh: "zh-CN",
-    en: "en-US",
-    ja: "ja-JP",
-    ko: "ko-KR",
-    vi: "vi-VN",
-    id: "id-ID",
-    ms: "ms-MY",
-  };
-  return locales[lang] ?? "en-US";
+function pad2(n: number): string {
+  return String(n).padStart(2, "0");
 }
 
-function formatDate(iso: string, lang: string): string {
+function formatDate(iso: string): string {
   const d = new Date(iso);
-  return new Intl.DateTimeFormat(localeFor(lang), {
-    year: "numeric",
-    month: lang === "en" ? "short" : "numeric",
-    day: "numeric",
-  }).format(d);
+  return `${d.getFullYear()}/${d.getMonth() + 1}/${d.getDate()}`;
 }
 
-export function formatTime(iso: string, lang: string): string {
+export function formatTime(iso: string): string {
   const d = new Date(iso);
-  return new Intl.DateTimeFormat(localeFor(lang), {
-    hour: "numeric",
-    minute: "2-digit",
-    hour12: lang === "en",
-  }).format(d);
+  return `${pad2(d.getHours())}:${pad2(d.getMinutes())}`;
 }
 
-export function formatDateTime(iso: string, lang: string): string {
-  return `${formatDate(iso, lang)} ${formatTime(iso, lang)}`;
+export function formatDateTime(iso: string): string {
+  return `${formatDate(iso)} ${formatTime(iso)}`;
 }