feat: finish the homepage plugin, move service links and canvas out of core (B19)

plugins/homepage/src/frontend/clock-timezone.ts

@@ -0,0 +1,28 @@
+/** IANA zone names use underscores, but people type and paste spaces. */
+export function normalizeTimezone(timezone: string): string {
+  return timezone.trim().replace(/\s+/g, "_");
+}
+
+export function isValidTimezone(timezone: string): boolean {
+  try {
+    new Intl.DateTimeFormat("en-US", { timeZone: timezone });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Normalizes the timezone a user typed and reports whether it is usable.
+ * A blank field is valid: it means "use local time".
+ */
+export function validateClockTimezone(timezone: unknown): {
+  timezone?: string;
+  valid: boolean;
+} {
+  if (typeof timezone !== "string" || timezone.trim() === "") {
+    return { timezone: undefined, valid: true };
+  }
+  const normalized = normalizeTimezone(timezone);
+  return { timezone: normalized, valid: isValidTimezone(normalized) };
+}