feat: Responsive device context in Stac templates

[Roshan-Dabhade]

Description

Currently, STAC templates do not have a built-in way to adapt styling or layout based on screen size or device type. Adding a layout-aware responsive context (mobile / tablet / web or width-based breakpoints) would allow templates to declaratively control spacing, typography and structure without requiring custom Flutter code, enabling true responsive SDUI across mobile, tablet and web.

Additional Context

Example context-

{
"type": "container",
"padding": {
"mobile": 8,
"tablet": 16,
"web": 32
},
"child": {
"type": "text",
"data": "Responsive STAC UI",
"style": {
"fontSize": {
"mobile": 12,
"tablet": 14,
"web": 18
}
}
}
}