fix(mobile): keep initial timeline loading honest

[judomanmk19xx-bot]

Summary

• keep the first channel timeline in loading state while the relay session has not connected
• preserve last-known messages across later reconnects
• add a regression test for the disconnected-before-first-load path

Problem

ChannelMessagesNotifier returned AsyncData([]) whenever the relay was not connected and no history had ever loaded. The UI could therefore render a synthetic empty channel even though message state was still unknown.

Verification

• dart format --output=none --set-exit-if-changed on both changed files
• dart analyze on both changed files: no issues
• full flutter test: 1,262 tests passed at 1d0984ff9f0ad36b023cf9085e6f780862fdbfc2
• git diff --check origin/main...HEAD

Manual testing

No signed iOS build or physical-device run was available locally, so device latency and reliability remain unmeasured. Expected device behavior: before the first successful history load, a disconnected session shows the existing loading skeleton rather than an empty timeline. After any successful load, reconnects retain the last-known messages.

Related work

• mobile: messages stop arriving after brief background/screen-lock until app is force-killed (zombie socket on resume) #4962 and fix(mobile): probe socket liveness on resume #5118 address a separate half-open socket problem after short iOS resumes. This PR is intentionally independent and changes only the first-load state contract.

No matching open PR was found for this fake-empty initial timeline state.