Tapping a mobile channel message with no replies currently opens an
empty thread. Only enable message-row thread navigation when the
existing thread summary reports replies, including replies known to the
relay but not loaded locally. Long-press → Reply still starts a new
thread.
Added widget tests for zero replies, loaded replies, relay-only replies,
and starting a thread through Reply. The zero-reply test failed before
the fix and passes afterward.
Validation: all 227 channel-page tests, the full mobile pre-push checks,
and all PR CI checks passed. The full repository gate failed in
unchanged Rust ACP timing tests (idle reset, keepalive, and steering
deadlines); this PR changes only mobile Dart files.
No duplicate open PR or issue found.
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Signed-off-by: Tom Brow <tomb@block.xyz>
