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/config.ts

@@ -2,12 +2,14 @@ import os from 'os';
 import path from 'path';
 
 import { readEnvFile } from './env.js';
+import { isValidTimezone } from './timezone.js';
 
 // Read config values from .env (falls back to process.env).
 const envConfig = readEnvFile([
   'ASSISTANT_NAME',
   'ASSISTANT_HAS_OWN_NUMBER',
   'ONECLI_URL',
+  'TZ',
 ]);
 
 export const ASSISTANT_NAME =
@@ -67,7 +69,17 @@ export const TRIGGER_PATTERN = new RegExp(
   'i',
 );
 
-// Timezone for scheduled tasks (cron expressions, etc.)
-// Uses system timezone by default
-export const TIMEZONE =
-  process.env.TZ || Intl.DateTimeFormat().resolvedOptions().timeZone;
+// Timezone for scheduled tasks, message formatting, etc.
+// Validates each candidate is a real IANA identifier before accepting.
+function resolveConfigTimezone(): string {
+  const candidates = [
+    process.env.TZ,
+    envConfig.TZ,
+    Intl.DateTimeFormat().resolvedOptions().timeZone,
+  ];
+  for (const tz of candidates) {
+    if (tz && isValidTimezone(tz)) return tz;
+  }
+  return 'UTC';
+}
+export const TIMEZONE = resolveConfigTimezone();

src/timezone.test.ts

@@ -1,6 +1,10 @@
 import { describe, it, expect } from 'vitest';
 
-import { formatLocalTime } from './timezone.js';
+import {
+  formatLocalTime,
+  isValidTimezone,
+  resolveTimezone,
+} from './timezone.js';
 
 // --- formatLocalTime ---
 
@@ -26,4 +30,44 @@ describe('formatLocalTime', () => {
     expect(ny).toContain('8:00');
     expect(tokyo).toContain('9:00');
   });
+
+  it('does not throw on invalid timezone, falls back to UTC', () => {
+    expect(() =>
+      formatLocalTime('2026-01-01T00:00:00.000Z', 'IST-2'),
+    ).not.toThrow();
+    const result = formatLocalTime('2026-01-01T12:00:00.000Z', 'IST-2');
+    // Should format as UTC (noon UTC = 12:00 PM)
+    expect(result).toContain('12:00');
+    expect(result).toContain('PM');
+  });
+});
+
+describe('isValidTimezone', () => {
+  it('accepts valid IANA identifiers', () => {
+    expect(isValidTimezone('America/New_York')).toBe(true);
+    expect(isValidTimezone('UTC')).toBe(true);
+    expect(isValidTimezone('Asia/Tokyo')).toBe(true);
+    expect(isValidTimezone('Asia/Jerusalem')).toBe(true);
+  });
+
+  it('rejects invalid timezone strings', () => {
+    expect(isValidTimezone('IST-2')).toBe(false);
+    expect(isValidTimezone('XYZ+3')).toBe(false);
+  });
+
+  it('rejects empty and garbage strings', () => {
+    expect(isValidTimezone('')).toBe(false);
+    expect(isValidTimezone('NotATimezone')).toBe(false);
+  });
+});
+
+describe('resolveTimezone', () => {
+  it('returns the timezone if valid', () => {
+    expect(resolveTimezone('America/New_York')).toBe('America/New_York');
+  });
+
+  it('falls back to UTC for invalid timezone', () => {
+    expect(resolveTimezone('IST-2')).toBe('UTC');
+    expect(resolveTimezone('')).toBe('UTC');
+  });
 });

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',