feat: implement Assets page with Carbon Design System

[samar-703]

What does this PR change?

• Implements the Assets page for OpenCost UI using Carbon Design System
• Adds summary metrics dashboard with 4 key metrics (Total Assets, Total Cost, Asset Types, Average Cost)
• Implements cost breakdown visualization with interactive donut chart
• Creates searchable and sortable data table with asset details (Name, Type, Provider, Cluster, CPU, RAM, Cost)
• Adds time window and asset type filtering
• Implements skeleton loading states for improved perceived performance
• Uses card-based layout with surface elevation for visual hierarchy

Does this PR relate to any other PRs?

• Part of the broader OpenCost UI revamp initiative
• This PR addresses the LFX mentorship coding challenge

How will this PR impact users?

• New functionality: Users can now view and analyze their infrastructure assets through the OpenCost UI
• Cost visibility: Users can quickly identify which asset types (Nodes, Disks, LoadBalancers, ClusterManagement) consume the most resources
• Data exploration: Searchable and sortable table allows users to drill down into specific assets
• Flexible analysis: Time window filtering enables users to analyze costs across different time periods
• Better UX: Skeleton loading states provide immediate feedback, making the application feel more responsive
• Improved discoverability: Tooltips and clear visual hierarchy help users navigate the interface

Does this PR address any GitHub or Zendesk issues?

• Closes Support Assets in the UI #28 (Implement Assets page)
• Related to [LFX Mentorship Term 1 2026] OpenCost UI Revamp #155 (LFX Mentorship Term 1 2026 - OpenCost UI Revamp coding challenge)

How was this PR tested?

• Tested against the demo API: https://demo.infra.opencost.io/model/assets
• Verified time window filtering works correctly (Today, Last 7 days, Last 30 days)
• Verified asset type filtering correctly filters the data table
• Tested table sorting functionality on all columns
• Tested search functionality filters assets by name
• Tested refresh button successfully updates data
• Verified skeleton loading states display correctly during data fetch
• Tested responsive behavior on different screen sizes
• Verified no console errors or warnings
• Tested with both populated data and edge cases (empty states, loading states)
• Confirmed all Carbon Design System components render properly

Does this PR require changes to documentation?

• No documentation changes required. The Assets API endpoint is already documented in the OpenCost/Kubecost documentation.

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?

• This PR is submitted as part of the LFX Mentorship coding challenge for Issue [LFX Mentorship Term 1 2026] OpenCost UI Revamp #155
• The maintainers can determine the appropriate release timeline and labeling based on the mentorship program schedule and their release roadmap
• Will defer to maintainer judgment on when this should be included in a release

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