Add Assets page to visualise infrastructure costs and utilisation

[Vrittigyl]

What does this PR change?

• Implements the Assets page using Carbon Design System components
• Adds cost visualization with donut charts and progress bars
• Creates a savings opportunities panel that highlights idle/wasted resources
• Implements expandable table rows for detailed asset information
• Adds support for multiple asset types (Node, Disk, LoadBalancer, Network, ClusterManagement)

Does this PR relate to any other PRs?

No

How will this PR impact users?

• Users can now view infrastructure asset costs in a clean, modern interface
• Idle resource alerts help users identify cost optimization opportunities immediately
• Visual breakdowns (charts, progress bars) make it easier to understand cost distribution
• Expandable rows provide detailed CPU/RAM utilization, cost breakdowns, and resource specs without cluttering the main view
• Time range filtering allows users to analyze costs over different periods

Does this PR address any GitHub issues?

• Part of Support Assets in the UI #28
• Mentorship coding challenge: [LFX Mentorship Term 1 2026] OpenCost UI Revamp #155

How was this PR tested?

• Manual testing with mock data covering all 5 asset types (Node, Disk, LoadBalancer, Network, ClusterManagement)
• Verified expandable table rows show correct breakdowns for each asset type
• Tested responsive layout on different screen sizes using Carbon's grid system
• Confirmed savings alerts only appear when idle percentage >= 70%
• Validated time range dropdown updates data correctly

UI Preview

Does this PR require changes to documentation?

No - this is a new page implementation following existing patterns in the codebase

Have you labeled this PR and its corresponding Issue as "next release" if it should be part of the next OpenCost release? If not, why not?

Yes, this addresses issue #28 which is part of the LFX mentorship coding challenge and should be included in the next release

[netlify]

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