feat: add Assets page with Carbon Design System (#28)

[taanvi2205]

What does this PR change?

• Adds a new Assets page to the OpenCost UI using the Carbon Design System (@carbon/react)
• Includes realistic mock/sample data (15 assets across GCP and AWS) that automatically loads when the OpenCost backend is unavailable, enabling local development and review without a running backend (npm run serve → http://localhost:1234/assets)
• Implements interactive donut chart (cost by asset type) and bar chart (cost by provider) with Recharts
• Adds summary tiles showing grand total and per-type cost breakdowns with click-to-filter
• Adds a sortable, searchable, paginated data table with color-coded asset type tags
• Adds an asset detail modal showing all properties, labels, and CPU/RAM utilization breakdown for nodes
• Includes Carbon Dropdown filters for time range, asset type, and category (URL-driven state)
• Auto-falls back to mock data when the OpenCost backend is unavailable for local development

Does this PR relate to any other PRs?

• No

How will this PR impact users?

• Users can now navigate to /assets from the sidebar to visualize their infrastructure assets (nodes, disks, load balancers, network, cluster management, cloud) with cost breakdowns, charts, and filtering

Does this PR address any GitHub or Zendesk issues?

• Addresses the Assets page support request (issue Support Assets in the UI #28)
• Part of the LFX Mentorship UI revamp coding challenge (issue [LFX Mentorship Term 1 2026] OpenCost UI Revamp #155)

How was this PR tested?

• Verified build passes with npm run build (Parcel)
• Tested locally at http://localhost:1234/assets using built-in sample data
• Confirmed all filters (time range, asset type, category) update URL params and re-filter data
• Confirmed summary tiles display correct totals and click-to-filter works
• Confirmed donut chart hover shows slice info in center, bar chart tooltip works
• Confirmed table sorting, search, pagination, and row-click detail modal all function
• Confirmed no CSS leaks to existing pages (Allocation, Cloud Costs, External Costs)

Does this PR require changes to documentation?

• No

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?

• Not labeled yet — this is a new feature contribution from the LFX Mentorship coding challenge

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