Fixes: Create dashboard layout primitives for admin panels

[wolfyres]

Closes #167

🎯 Context

This PR delivers issue 3 of 70 for the Demo Admin Dashboard initiative. It introduces foundational layout primitives designed to help maintainers populate and manage demo UI data. To ensure zero risk to the core product, all administrative tooling and layout components have been strictly isolated from production mail flows.

🛠️ Key Changes

Built AdminPage to serve as the root wrapper for admin views.

Built AdminPanel to act as a structured card container for forms and tables.

Built AdminToolbar to handle titles, filters, and action buttons.

Built AdminSection to manage spacing and logical grouping within panels.

Created an interactive preview component using fake, deterministic data for safe public repository review.

Confirmed all files are scoped exclusively to src/features/demo-admin-dashboard/.

✅ Acceptance Criteria Verified

[x] Work is implemented entirely under the demo-admin-dashboard folder.

[x] No files outside the target folder were touched.

[x] Primitives support the broader goal of populating demo data into the UI.

[x] Validation and preview documentation is included via a preview component.

[x] The PR is fully self-contained and reviewable without unrelated refactors.