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.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');
+  });
 });