docs: archive cleanup plan, update SPEC and README for v0.45–v0.47

- Archive docs/designs/2026-05-04-repo-cleanup-design.md to docs/archive/designs/
- Remove docs/designs/2026-05-04-settings-sidebar-demo.html (implemented)
- SPEC.md: add CardDAV Accounts and CardDAV Addressbook Selection tables
- SPEC.md: expand Contacts table with multi-value fields, CardDAV columns, and
  new contact_phones / contact_emails / contact_addresses sub-tables
- SPEC.md: add birthday reminder_offset columns (v0.46.0)
- SPEC.md: update External Calendars table (apple→caldav migration note)
- SPEC.md: update Tasks module (bulk actions, v0.42.0)
- SPEC.md: update Contacts module (CardDAV multi-account sync, multi-value fields)
- SPEC.md: update Birthdays module (flexible reminder offsets, v0.46.0)
- SPEC.md: update Settings module (Synchronization tab, module toggles, scheduled
  backups, CardDAV UI, correct tab count and names)
- README.md: update Birthdays row (flexible reminder offsets)
- README.md: update Backup row (automatic scheduled backups)
- .gitignore: exclude screenshot scripts, seed scripts, backup files,
  docs/github-pages/ staging folder

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

.gitignore

@@ -52,3 +52,16 @@ docs/repo-audit-*.md
 *.txt
 !public/robots.txt
 CLAUDE.md
+
+# Screenshot generation scripts (dev tooling, not part of the app)
+create-screenshots*.js
+custom-screenshots.js
+seed-demo-simple.js
+seed-english-demo*.js
+screenshots/
+
+# Database backup files
+*.db.backup-*
+
+# Duplicate GitHub Pages staging folder
+docs/github-pages/