feat: auto-save drafts, timezone picker, contract archival, staging env

[yosemite01]

Summary

Resolves issues #168, #170, #174, and #177 in a single branch.

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#168 — Auto-save create stream form drafts

Closes #168

• New hooks/use-form-draft.ts hook saves form state to localStorage on every change, debounced 500 ms.
• On page load, if a draft exists (and is less than 24 h old) a restore banner appears: Restore or Discard.
• Draft is cleared automatically on successful stream creation.
• clearExpiredDrafts() is called on mount to prune any stale entries.

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#170 — Timezone-aware date/time picker

Closes #170

• User's detected timezone (via Intl.DateTimeFormat) is displayed next to every date/time input as (UTC±N).
• A timezone selector dropdown lets users schedule in a different timezone.
• Replaced toISOString().slice(0, 16) in localDatetimeMin and addDuration with locale-safe local-time formatting so dates are displayed in the browser's local time, not UTC.
• Stream detail page now shows UTC time alongside local time for Start and End fields.

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#174 — Smart contract storage archival for completed streams

Closes #174

• Added ArchiveSentBy(Address) and ArchiveReceivedBy(Address) keys to DataKey.
• On cancel, stream IDs are moved from SentBy/ReceivedBy to the corresponding archive keys — active indexes stay lean.
• On withdraw, when a stream is fully drained after end_time, it is likewise moved to the archive.
• New read functions: get_archived_sent_streams and get_archived_received_streams (paginated).
• New cleanup_stream(caller, stream_id) lets either party reclaim storage for cancelled/completed streams (removes from all indexes and deletes the stream entry).

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#177 — Staging environment with separate testnet contract deployment

Closes #177

• Added .github/workflows/staging.yml:
  • Pushes to the staging branch deploy to Vercel with the staging contract (STAGING_CONTRACT_ID_TESTNET secret).
  • Every PR gets a Vercel preview deployment; the preview URL is posted as a PR comment.
• Updated .env.local.example with staging variable documentation and the NEXT_PUBLIC_APP_ENV flag.
• Required GitHub secrets: STAGING_CONTRACT_ID_TESTNET, VERCEL_TOKEN, VERCEL_ORG_ID, VERCEL_PROJECT_ID (configured in the staging environment in repo settings).

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Test plan

• Create stream form: fill fields, navigate away, reload — restore banner appears with correct timestamp
• Click Restore — fields repopulate; click Discard — banner dismisses and draft is gone
• Timezone dropdown shows detected tz; offset label updates next to each date input
• Stream detail page shows UTC time alongside local time for Start and End
• Contract: cancel a stream and verify the stream ID is no longer in get_sent_streams but appears in get_archived_sent_streams
• Contract: cleanup_stream removes data for a cancelled stream
• Push a commit to staging branch and verify Vercel deploys with staging contract ID

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