[Dashboard] Minimal UI-only Dashboard component + Storybook demo

[bharathkumar39293]

Implements a minimal, controlled Dashboard grid component for issue #94.

Features:

• Controlled via widgets, layout, and onLayoutChange.
• Drag/resizing handled by react-grid-layout.
• Supports per-widget constraints:
  • minSize / maxSize
  • movable / resizable (locking via movable: false)
• Optional className prop for theming/styling.
• Storybook demo included:
  • Default: two draggable/resizable button widgets.
  • Themed: same layout with custom className.

Notes:

• No persistence, roles, or sharing logic; left to consumers.
• TypeScript typings are included and compatible with the repo.
• Controlled, minimal, and mergeable; designed for easy future extension.

References issue #94.

[mohdsayed]

Closing as we discussed here: #94 (comment)