Description

Add ability to toggle any panel to fullscreen mode for focused work.

Feature Details

• Keybinding: F in NORMAL mode
• Behavior: Toggle current active panel to fullscreen
• Exit: Press F again or Esc to return to normal layout

Expected Behavior

Before (normal):                    After (fullscreen):
┌─────────┬─────────────┐          ┌─────────────────────┐
│ Collec  │  Request    │          │                     │
│         ├─────────────┤   F →    │    Request Panel    │
│         │  Response   │          │    (fullscreen)     │
└─────────┴─────────────┘          │                     │
                                   └─────────────────────┘

Technical Implementation

• Add fullscreenPanel state to Model
• Modify View() to render only active panel when fullscreen
• Update keybinding handler for F key
• Add visual indicator (e.g., [FULLSCREEN] in status bar)

Acceptance Criteria

• F toggles fullscreen for current panel
• All panels support fullscreen mode
• Status bar shows fullscreen indicator
• Esc or F exits fullscreen
• Keybindings remain functional in fullscreen