refactor(dashboard): replace hardcoded colors with theme tokens

[claytonlin1110]

Summary

• Refactors dashboard components to remove hardcoded color values and route styling through src/theme.ts.
• Replaces inline #... and rgba(...) usage with theme.palette, TEXT_OPACITY, and alpha(...)-based theme values.
• Adds reusable theme constants for shared dashboard visuals:
  • CONTRIBUTION_HEATMAP_SCALE
  • REPO_OWNER_AVATAR_BACKGROUNDS

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Scope

• src/components/dashboard/CommitTrendChart.tsx
• src/components/dashboard/ContributionHeatmap.tsx
• src/components/dashboard/GlobalActivity.tsx
• src/components/dashboard/KpiCard.tsx
• src/components/dashboard/LeaderboardCharts.tsx
• src/components/dashboard/LiveCommitLog.tsx
• src/components/dashboard/PRStatusChart.tsx
• src/components/dashboard/RepositoriesTable.tsx
• src/theme.ts

Why

• Improves consistency and maintainability of dashboard styling.
• Ensures color behavior is centrally controlled via theme tokens.
• Makes future design updates safer and faster by avoiding scattered hardcoded values.

Checklist

• New components are modularized/separated where sensible
• Uses predefined theme (e.g. no hardcoded colors)
• Responsive/mobile checked
• Tested against the test API
• npm run format and npm run lint:fix have been run
• npm run build passes
• Screenshots included for any UI/visual changes

[claytonlin1110]

@anderdc @LandynDev
Currently there are too many hard coded color value usages in the entire codebase, I'm starting to refactoring to get rid of hard coded values.
As literally all the files have hard coded values, I'm starting to refactor dashboard pages & components...
Would you please review this?