feat(assets): Add modern Assets page with dark/light theme toggle

[AAdIprog]

feat(assets): Add modern Assets page with dark/light theme toggle

• Add new Assets page with summary cards, pie chart, and data table
• Implement dark/light theme toggle on all pages
• Add sparkline visualizations for cost metrics
• Create ThemeContext for global theme state with localStorage persistence
• Update all existing pages with theme toggle button
• Add comprehensive CSS styling for both themes
• Mock data support for development without backend

What does this PR change?

• Adds a new Assets page (/assets) with modern UI components including summary metric cards with sparkline charts, a pie chart showing cost breakdown by asset type, and a styled data table with color-coded type badges
• Implements a dark/light theme toggle accessible from all pages via a button in the page header
• Creates a ThemeContext for global theme state management with localStorage persistence so user preference is remembered across sessions
• Updates CSS with comprehensive theming support using CSS variables for consistent styling in both modes

Does this PR relate to any other PRs?

• No, this is a standalone feature addition

How will this PR impact users?

• Users will have a new Assets page to view and analyze asset costs with modern visualizations
• Users can now toggle between dark and light themes across the entire application based on their preference
• The theme preference persists across sessions, improving user experience

Does this PR address any GitHub or Zendesk issues?

• Closes Support Assets in the UI #28

How was this PR tested?

• Manual testing with REACT_APP_USE_MOCK_DATA=true npm run dev
• Verified Assets page renders correctly with mock data
• Tested theme toggle functionality across all pages (Assets, Cost Allocation, Cloud Costs, External Costs)
• Confirmed theme persistence in localStorage across page refreshes
• Tested both dark and light modes for visual consistency

Does this PR require changes to documentation?

• No documentation changes required

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 should be labeled as "next release" as it adds significant new functionality

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