Drag-and-drop gauge layout with localStorage persistence

## Description
Let users rearrange gauges, chart panels, and data cards by dragging them. Save the custom layout to localStorage so it persists across sessions.

## Details
- Drag handles on each panel/gauge
- Snap-to-grid layout system
- Save/load named layouts
- Reset to default option
- Similar to Foxglove's panel system but simpler and more opinionated

## Priority
v1.3+