Add timestamp-based last-read tracking and unread divider markers

[Specter242]

Summary

Adds timestamp-based last-read tracking for contacts/channels and uses it to place an in-chat unread divider for DM and channel threads.

Why

Unread counts alone are not stable for locating the unread boundary after list mutations.
Timestamp markers provide deterministic first-unread positioning while preserving count-based fallback behavior.

Changes

• lib/storage/unread_store.dart
  • Persist/debounce contact and channel last-read timestamps.
• lib/connector/meshcore_connector.dart
  • Store/load in-memory last-read maps.
  • Add timestamp getters:
    • contactLastReadTimestamp(...)
    • channelLastReadTimestamp(...)
  • Add first-unread helpers with fallback:
    • firstUnreadContactIndex(...)
    • firstUnreadChannelIndex(...)
  • Update read-mark APIs to persist timestamps.
  • Remove marker state when deleting contacts/channels.
• lib/screens/chat_screen.dart
  • Render unread divider for DM threads.
• lib/screens/channel_chat_screen.dart
  • Render unread divider for channel threads.
• lib/widgets/unread_marker_divider.dart
  • Shared unread divider widget.

Behavior Notes

• Divider placement prefers timestamp markers.
• If no marker resolves to a newer message, logic falls back to unread-count behavior.
• Existing unread counters remain intact.

Validation

• dart format on touched files
• flutter analyze lib/connector/meshcore_connector.dart lib/storage/unread_store.dart lib/screens/chat_screen.dart lib/screens/channel_chat_screen.dart lib/widgets/unread_marker_divider.dart (pass)

Issue

Part of #73

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fall back to unread count when marker scan finds nothing

When a last-read marker exists, this branch returns null if no message timestamp is greater than the marker, so the count-based fallback is never used. Because markContactRead/markChannelRead store DateTime.now() while incoming message timestamps are device-provided, clock skew or unset device time can leave unread messages with timestamps <= marker; in that case unread counts stay positive but first-unread positioning is lost (same logic is duplicated in firstUnreadChannelIndex).

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[Specter242]

Addressed in 048b798.\n\nI updated both firstUnreadContactIndex and firstUnreadChannelIndex to always fall back to unread-count positioning when a timestamp marker exists but no message is newer than that marker. This keeps first-unread placement working under clock skew / device-time mismatch cases while still preferring marker-based placement when available.