Add plan dates, postponing, backlog and projects

- Tasks get an optional plan date separate from the deadline. Unfinished
  plans roll over in Today; only missed deadlines count as overdue.
- Postpone sheet on every task (tomorrow, weekend, next week, ...) that
  can move either the plan date or the deadline, with undo.
- Backlog view for undated tasks, with suggestions in Today.
- Projects with deadline, notes, progress, next step and "falling
  behind" warnings, shown in Today and in their own view.
- Times and the calendar now follow the plan date (else the deadline).
- Schema v3 migration (backup taken first); export format v3 includes
  projects and still imports older files.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/static/sw.js

@@ -1,5 +1,5 @@
 // Network-first service worker: always fresh when online, app shell still opens offline.
-const CACHE = 'whattodo-v2';
+const CACHE = 'whattodo-v3';
 const SHELL = [
   './', 'index.html', 'style.css', 'app.js', 'manifest.webmanifest',
   'icons/icon.svg', 'icons/icon-192.png', 'icons/icon-512.png',