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>

package.json

@@ -23,12 +23,12 @@
   "dependencies": {
     "@onecli-sh/sdk": "^0.2.0",
     "better-sqlite3": "^11.8.1",
-    "grammy": "^1.39.3",
     "cron-parser": "^5.5.0",
     "pino": "^9.6.0",
     "pino-pretty": "^13.0.0",
     "yaml": "^2.8.2",
-    "zod": "^4.3.6"
+    "zod": "^4.3.6",
+    "grammy": "^1.39.3"
   },
   "devDependencies": {
     "@eslint/js": "^9.35.0",