docs: update SPEC, CONTRIBUTING, README for v0.3.0 and v0.4.0 features

- SPEC.md: add notes full-text search, contacts vCard import/export,
  recurrence_parent_id + budget_recurrence_skipped table, sync_config table
- CONTRIBUTING.md: fix test file location (project root, not tests/),
  add offline.html to project structure
- README.md: Node.js ≥22, Apple CalDAV UI setup (Option A), fix
  Contributing section link, update Notes/Contacts feature highlights

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

CONTRIBUTING.md

@@ -69,7 +69,8 @@ public/
   components/          # Reusable Web Components (oikos-* prefix)
   pages/               # Page modules — each exports a render() function
   sw.js                # Service worker
-tests/                 # One test file per module
+  offline.html         # Offline fallback page (served by service worker)
+test-[module].js       # One test file per module (project root)
 docs/                  # Product spec, screenshots
 ```
 
@@ -195,7 +196,7 @@ PRs are reviewed by the maintainer. Expect feedback within a few days. Once appr
 
 ### Testing
 
-- One test file per module in `tests/`
+- One test file per module in the project root (`test-[module].js`)
 - Tests use in-memory SQLite via `--experimental-sqlite`
 - Import route handlers directly — no HTTP calls, no running server