Port channel windows to mobile

[wesbillman]

Summary

• port mobile channel timelines to the desktop server-assembled channel-window path from GUI read-model overhaul: server-assembled channel windows (Correct™ pagination + relay-signed bounds) #1500
• add authed HTTP /query support with NIP-98 auth, NIP-CW parsing/store behavior, relay thread summaries, and thread subtree fetching
• keep websocket history fallback and make window pagination retry-safe on page fetch/parse failures

Validation

• flutter analyze
• flutter test test/shared/relay/relay_session_test.dart test/shared/relay/nostr_models_test.dart test/features/channels/channel_window_test.dart test/features/channels/channel_messages_provider_test.dart
• flutter test
• iOS simulator smoke on iPhone 17 Pro (17D22AD8-E949-42DF-B85B-4E26B5558DBB) against staging relay via:
  • flutter run -d 17D22AD8-E949-42DF-B85B-4E26B5558DBB --dart-define=BUZZ_RELAY_URL=https://sprout-oss.stage.blox.sqprod.co
  • opened #general
  • scrolled back from Jul 5/Jul 3 to Jun 28/29, exercising real cursor paging
  • confirmed no ChannelMessagesNotifier window fallback/error logs during channel open/scroll/thread smoke
  • confirmed thread chips/reactions rendered while paged back
  • sent a live smoke message and saw it appear in simulator
  • opened a 3-reply thread and saw thread subtree content load

Smoke evidence

Scroll-back deep in #general:

Live message appeared after send:

Thread opened/loaded:

Notes

• Live WS fallback/downgrade was not forced in the simulator; fallback behavior is covered in provider tests.
• The simulator video recording attempt produced a 0-byte file, so screenshots + logs are attached as smoke evidence instead.

[wesbillman]

Conformance review (Brain): reviewed against the NIP-CW porting contract (checklist A–H derived from #1500 + docs/nips/NIP-CW.md) — details in the Buzz mobile-relay-updates thread. Verified independently: parser/store semantics (39006 as sole exhaustion authority, d-tag request binding, composite-cursor interval asserts with the correct id-ascending tiebreak, immutable page chain, overlap rejection), NIP-98 /query transport incl. exact-body payload hash + nonce, thread-subtree fetch replacing soup-derived replies, WS downgrade fallback retained, retry-safe window pagination. flutter analyze clean + full suite (426 pass / 1 skip) re-run locally; head SHA matches the reviewed working tree exactly. LGTM — approval can't come from this account since it opened the PR.