fix: validate timezone to prevent crash on POSIX-style TZ values

POSIX-style TZ strings like IST-2 cause a hard RangeError crash in
formatMessages because Intl.DateTimeFormat only accepts IANA identifiers.

- Add isValidTimezone/resolveTimezone helpers to src/timezone.ts
- Make formatLocalTime fall back to UTC on invalid timezone
- Validate TZ candidates in config.ts before accepting
- Add timezone setup step to detect and prompt when autodetection fails
- Use node:22-slim in Dockerfile (node:24-slim Trixie package renames)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/timezone.ts

@@ -1,11 +1,32 @@
+/**
+ * Check whether a timezone string is a valid IANA identifier
+ * that Intl.DateTimeFormat can use.
+ */
+export function isValidTimezone(tz: string): boolean {
+  try {
+    Intl.DateTimeFormat(undefined, { timeZone: tz });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Return the given timezone if valid IANA, otherwise fall back to UTC.
+ */
+export function resolveTimezone(tz: string): string {
+  return isValidTimezone(tz) ? tz : 'UTC';
+}
+
 /**
  * Convert a UTC ISO timestamp to a localized display string.
  * Uses the Intl API (no external dependencies).
+ * Falls back to UTC if the timezone is invalid.
  */
 export function formatLocalTime(utcIso: string, timezone: string): string {
   const date = new Date(utcIso);
   return date.toLocaleString('en-US', {
-    timeZone: timezone,
+    timeZone: resolveTimezone(timezone),
     year: 'numeric',
     month: 'short',
     day: 'numeric',