Add quick pane zoom icons in the pane toolbar

[guysmoilov]

Problem

Zooming a pane currently requires tmux commands/keyboard flow, which is slow on mobile.

Proposal

Add explicit zoom/unzoom icon buttons in the pane controls so users can quickly toggle pane zoom with touch.

Scope

• Show a zoom action when pane is not zoomed
• Show an unzoom action when pane is currently zoomed
• Wire action through existing control plane message path
• Keep icon state synced with tmux state updates

Acceptance Criteria

• Tapping zoom icon zooms active pane immediately
• Tapping unzoom icon restores layout
• Icon state always matches pane.isZoomed state
• Covered by tests for control message handling and UI state updates