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>

.env.example

@@ -0,0 +1,17 @@
+# Copy to .env and fill in your values
+
+# ntfy topic — make it unguessable (anyone who knows it can receive/send)
+# Subscribe at https://ntfy.sh/YOUR_TOPIC or in the ntfy app
+NTFY_TOPIC=your-unguessable-topic-here
+
+# Optional: self-hosted ntfy server
+# NTFY_URL=https://ntfy.sh
+
+# Optional: URL the notification links back to (your deployed app)
+# APP_URL=http://localhost:8080
+
+# Optional: cron schedule (default: 9 AM on the 1st of every month)
+# CRON_SCHEDULE=0 9 1 * *
+
+# Set to "true" to get a test ping the moment the notifier starts
+# NTFY_TEST_ON_START=false