feat: Add colored frames/zones for visual separation

[camilocbarrera]

Summary

Add resizable colored frames/zones to visually group and organize cards on the canvas.

Problem

Large boards become cluttered. Users need a way to visually separate different areas (e.g., categories, stages, teams) without relying solely on card positioning.

Proposed Solution

• Rectangular frames that sit behind cards
• Customizable background color with low opacity
• Optional title/label at the top
• Resizable via drag handles
• Cards can be placed inside frames (visual only, no grouping logic needed initially)

Technical Approach

1. Add frames table: id, sessionId, title, color, x, y, width, height, zIndex
2. Render frames below cards layer (lower z-index)
3. Add frame CRUD actions and realtime sync
4. Add "Create Frame" option to command menu or toolbar

Acceptance Criteria

• Create, resize, move, and delete frames
• Color picker for frame background
• Optional frame title
• Frames render behind cards
• Realtime sync across participants