[Feature] Add Zen mode for distraction-free editing

[kom50]

Summary

Full-screen, minimal UI mode for focused coding.

Requirements

• Toggle with button or F11
• Hide header, user list, all UI chrome
• Center editor with max-width
• Subtle fade effect on transition
• Press Escape to exit
• Optional: hide line numbers

UI/UX

• Smooth fade transition (0.3s)
• Slightly dimmed background
• Floating exit hint at bottom

Acceptance Criteria

• Zen mode hides all distractions
• Editor is centered and focused
• Easy to enter and exit