feat: richer link preview cards and user local time display

P2-7: Domain-specific URL preview cards — YouTube shows mqdefault.jpg
thumbnail with ▶ play overlay + title; GitHub shows inline SVG icon +
owner/repo parsed from og:title + star/language info from og:description;
generic cards get a Google favicon when og:image is absent; empty cards
(no title or description) are suppressed entirely

P2-9: Live local time in user profiles — useLocalTime(timezone, hour12)
uses Intl.DateTimeFormat with the user's m.tz IANA zone; updates every
60s; shows clock icon + formatted time + timezone abbreviation (EST/JST
etc.) in dim text; respects existing hour24Clock setting

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/hooks/useLocalTime.ts

@@ -0,0 +1,59 @@
+import { useCallback, useEffect, useState } from 'react';
+
+function formatLocalTime(timezone: string, hour12: boolean): string | undefined {
+  try {
+    return new Intl.DateTimeFormat('en', {
+      timeZone: timezone,
+      hour: 'numeric',
+      minute: '2-digit',
+      hour12,
+    }).format(new Date());
+  } catch {
+    return undefined;
+  }
+}
+
+function getTimezoneAbbr(timezone: string): string | undefined {
+  try {
+    return new Intl.DateTimeFormat('en', {
+      timeZone: timezone,
+      timeZoneName: 'short',
+    })
+      .formatToParts(new Date())
+      .find((p) => p.type === 'timeZoneName')?.value;
+  } catch {
+    return undefined;
+  }
+}
+
+export type LocalTimeInfo = {
+  time: string;
+  abbr: string | undefined;
+};
+
+/**
+ * Returns the current time (and timezone abbreviation) in the given IANA
+ * timezone, updated every minute. Returns undefined when timezone is
+ * undefined or invalid.
+ */
+export function useLocalTime(
+  timezone: string | undefined,
+  hour12: boolean = true,
+): LocalTimeInfo | undefined {
+  const compute = useCallback((): LocalTimeInfo | undefined => {
+    if (!timezone) return undefined;
+    const time = formatLocalTime(timezone, hour12);
+    if (!time) return undefined;
+    return { time, abbr: getTimezoneAbbr(timezone) };
+  }, [timezone, hour12]);
+
+  const [info, setInfo] = useState<LocalTimeInfo | undefined>(compute);
+
+  useEffect(() => {
+    setInfo(compute());
+    const id = window.setInterval(() => setInfo(compute()), 60_000);
+    return () => window.clearInterval(id);
+  }, [compute]);
+
+  return info;
+}