docs: add offline collection guide and example

[dhruvxvaishnav]

Closes #10

Summary

• Document how the current Supabase collection adapter composes with compatible browser SQLite persistence and offline transaction packages.
• Add a browser-only todos example that keeps the collection cache in OPFS SQLite and queues insert, update, and delete actions for PostgREST replay.
• Explain Auth/RLS behavior, retry and idempotency limits, Realtime recovery, conflicts, and multi-tab leadership. No adapter source changes were needed for this experiment.

Validation

• pnpm exec vitest run --project unit tests/offline-example.test.ts: 5 passed.
• pnpm test: 272 passed, 4 todo.
• pnpm typecheck: passed.
• pnpm build: passed.
• pnpm test:e2e in WSL with local Supabase: 48 passed.
• pnpm check on a Linux copy with LF line endings: passed with four pre-existing suppression warnings in src/db.ts. The Windows checkout check reported 39 formatting errors from CRLF line endings in unchanged files.
• Browser smoke test on localhost: offline insert, update, and delete each survived a reload; reconnect replay sent POST, PATCH, then DELETE against mocked PostgREST responses and drained the outbox. This did not exercise a live server's Auth, RLS, Realtime, or conflict behavior.
• git diff --check: passed.