Extract shared chain-waiting utilities to reduce E2E and Playwright test duration/flakiness

Context

Raised in PR #129 review comment.

The polling-based wait helpers in examples/papi/common.js (waitForBlock, waitForAgreementAcceptance, etc.) are a source of slowness and potential flakiness in both the PAPI E2E test suite and the UI Playwright E2E tests.

Problem

• Polling is slow: waitForAgreementAcceptance polls chain state every few seconds with a 60s timeout. With 2s dev-mode blocks, this adds unnecessary latency.
• Shared pain: The UI Playwright tests face the same issue — they also need to wait for on-chain state changes (agreement accepted, bucket created, etc.) and likely use similar polling patterns.
• Flakiness risk: Polling with fixed timeouts can flake under CI load. Event-based waiting would be more deterministic.

Proposed improvements

1. Event-based waiting — subscribe to finalized block events and filter for the expected pallet event (e.g. StorageProvider::AgreementAccepted) instead of polling storage. Faster and more reliable.
2. Shorter block times for Playwright — if Playwright tests don't already use --dev-block-time, switching from Zombienet (6s blocks + finality lag) to dev mode (2s instant-finalized blocks) would cut wait times significantly.
3. Shared test-helpers package — extract waitForBlock, waitForAgreementAcceptance, and similar utilities into a shared package that both PAPI E2E and Playwright tests consume (related to #135).

Acceptance criteria

• Audit current wait patterns in E2E and Playwright tests
• Implement event-subscription-based waiting where possible
• Evaluate dev-mode chain for Playwright tests
• Measure before/after test suite duration