In multi-deck mode, Messages column now uses stacked navigation instead
of side-by-side split pane. Full-width contact list OR full-width chat —
clicking a conversation navigates to chat, back arrow returns to list.
Single-pane mode keeps the existing split layout (280dp list + flex chat).
Changes:
- Add compactMode param to DesktopMessagesScreen (default false)
- Extract SplitMessagesContent and CompactMessagesContent composables
- Add onBack callback to ChatPane with back arrow in header
- Remove hardcoded 280dp from ConversationListPane (caller controls width)
- Pass compactMode=true from deck RootContent
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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