Add 'Recent Panes' section at top of sidebar (default N=3)

[guysmoilov]

Problem

Switching between a few actively used panes takes too many taps in larger tmux layouts.

Proposal

Add a Recent Panes section at the very top of the sidebar that lists the last N focused panes, with default N=3.

Scope

• Track pane focus history in control-plane/UI state
• Show unique recent panes ordered by recency (most recent first)
• Pin section at top of sidebar above other pane/session lists
• Make default history size 3 (configurable later if desired)

Behavior Details

• Do not duplicate the same pane multiple times in list
• Re-focusing a pane moves it to the top
• If a pane closes, remove it from recent list
• Clicking a recent pane focuses/selects it

Acceptance Criteria

• Sidebar shows Recent Panes at top
• By default, at most 3 panes are shown
• Ordering reflects last focus events accurately
• Closed panes are pruned from the list
• Add tests for recency ordering and deduplication behavior