Replace phone tree with grouped session list

- Phone layout uses grouped ItemsControl instead of TreeView
- Agent names (codex, cursor, copilot) are group headers
- Tapping header expands/collapses sessions via IsExpanded binding
- TreeItemTapped now also sets SelectedNode for leaf items
- Removed NavigationView dependency from PhoneMainView

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sharpninja