Studio: free-form layout editor

[fberrez]

Beyond the 5 fixed layouts — a drag/resize grid so users place slots freely on the 800x480 canvas.

Scope

• Grid-based slot placement (snap, resize) producing a layout the renderer + docs/layout-schema.md can consume.
• Keep the curated layouts as one-click starting points.
• Server-side validation that slots fit the canvas and don't overlap illegally.

Notes

• Bigger build; competes with DAKboard's editor. User signal asked for "show what I want on it". Likely an epic — split into schema, editor UI, and validation sub-tasks.