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>

docker-compose.yml

@@ -6,6 +6,21 @@ services:
       - "8080:80"
     restart: unless-stopped
 
+  notifier:
+    build: ./notifier
+    restart: unless-stopped
+    environment:
+      # Required: set your ntfy topic (keep it unguessable, e.g. "nate-letter-k7x2")
+      NTFY_TOPIC: ${NTFY_TOPIC}
+      # Optional overrides
+      NTFY_URL: ${NTFY_URL:-https://ntfy.sh}
+      APP_URL: ${APP_URL:-http://localhost:8080}
+      CRON_SCHEDULE: ${CRON_SCHEDULE:-0 9 1 * *}   # 9 AM on the 1st of every month
+      # Set to "true" to get a test ping immediately on container start
+      NTFY_TEST_ON_START: ${NTFY_TEST_ON_START:-false}
+    depends_on:
+      - app
+
   dev:
     image: node:20-alpine
     working_dir: /app