Fix Docker build error; add multi-year view, PDF export, ntfy notifier

Fix: remove unused 'accessibleMonths' variable (tsc exit code 2)

Features:
- AllYearsLetter view: all years in chronological order, reachable
  from "All Years" at the top of the sidebar
- PDF export: window.print() with @media print CSS — sidebar, toolbar,
  and chrome hidden; each year starts on a new page via page-break
- "Download all" exports a plain-text file of every year's letter
- ntfy notifier: standalone Node.js service (notifier/) that fires a
  push notification via ntfy on the 1st of each month; configurable
  via NTFY_TOPIC, NTFY_URL, CRON_SCHEDULE, APP_URL env vars
- docker-compose: adds `notifier` service alongside `app`; reads from
  .env (example in .env.example); NTFY_TEST_ON_START=true sends a
  test ping on container start

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

notifier/package.json

@@ -0,0 +1,9 @@
+{
+  "name": "letter-notifier",
+  "version": "1.0.0",
+  "type": "module",
+  "main": "index.mjs",
+  "dependencies": {
+    "node-cron": "^3.0.3"
+  }
+}