fix(frontend): improve chart contrast in dark mode

[nehaaprasaad]

Summary

• Improves readability of chart titles and series colors in dark mode. Light mode and PNG chart export behavior stay sensible (title fallback on export).

Changes

• Theme-aware SVG chart title fill with a safe fallback when CSS variables are not available.
• Dark palette overrides for --chart-1–--chart-5 so bars and other series colors are not too dark on the dark background.
• Explicit text-foreground on the chart toolbar title so it does not inherit muted parent text.

Files

apps/shared/src/chart-builder.tsx
apps/frontend/src/styles.css
apps/frontend/src/components/tool-calls/display-chart.tsx

fix : #667

[cubic-dev-ai]

No issues found across 3 files

[github-actions]

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[Bl3f]

lgtm