fix(mobile): keep just-created channels visible while membership provisions

[arthur-albuquerque]

Fixes #7780

Problem

Channels created inside a project (and any other channel created from another client) never appeared in the iOS channel list. The mobile list is built from two sources — the user's kind:39002 memberships and the open-channel directory — and the relay provisions the creator's kind:39002 membership asynchronously after kind:9007. A mobile refresh that lands in that window sees neither a membership entry nor (for private channels) a directory entry, so the channel is dropped from the list permanently. Desktop is unaffected because AppState::pending_owned_channels overlays exactly this gap (desktop/src-tauri/src/app_state_pending_channels.rs).

Change

Mirror Desktop's pending-owner overlay on mobile:

• ChannelsNotifier.markPendingOwnedChannel records the channel id right after the successful kind:9007 submission in ChannelActions.createChannel, keyed by relay-and-identity scope.
• _fetchChannels treats pending ids as members: they are added to the metadata-fetch set and classified isMember: true, so the channel stays listed.
• Each pending id is cleared as soon as real kind:39002 membership is observed, so a later leave still flips is_member back to false and the overlay never masks a genuine removal.
• A community or identity switch clears the overlay with the other scope-scoped state, so stale entries can never classify channels in a new scope.

Scope note

This fixes the visibility window that made project channels unreachable on iOS. A full Projects surface on mobile (kind:30621 read models, the buzz-channel/buzz-related-channel bindings) is a separate feature and is intentionally not attempted here.

Testing

• flutter analyze — no issues
• dart format --set-exit-if-changed — clean
• node mobile/scripts/check-file-sizes.mjs — clean
• flutter test test/features/channels/channels_provider_test.dart — 74 passed, including 3 new regression tests:
  1. a pending channel with no kind:39002 entry stays listed with isMember: true, and its metadata is fetched;
  2. the overlay clears once real membership lands, so a later membership removal drops the channel (no permanent mask);
  3. the overlay does not leak across an identity switch.
• Full flutter test test/features/channels/ — 1060 passed.
• Full flutter test shows 15 pre-existing failures in test/shared/push/push_presentation_export_test.dart (native MethodChannel mocks); reproduced identically on a clean checkout of ef2aa1a with no changes — unrelated to this PR.

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