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();