feat(ui): redesign light and dark theme for cohesion and accessibility

[w7-mgfcode]

Summary

Audit and redesign the ForecastLabAI dashboard's light and dark themes across every route for a cohesive, accessible, professionally polished look-and-feel.

Scope

• Full sweep of every route under frontend/src/pages/ in both light and dark mode.
• Redesign the theming layer: light/dark semantic-token palette (OKLCH) in frontend/src/index.css.
• Replace raw Tailwind color utilities and manual dark: overrides with shadcn semantic tokens.

Acceptance criteria

• Both themes audited per-route with screenshots; defect inventory produced.
• Redesigned light + dark palette applied via semantic tokens.
• WCAG AA contrast: body text >= 4.5:1, large/UI >= 3:1, in both themes.
• No raw color utilities / no manual dark: color overrides remain.
• pnpm tsc --noEmit && pnpm lint && pnpm test --run green.
• All existing functionality and layout preserved.

Constraints

Frontend-only. shadcn work goes through the shadcn skill + MCP per .claude/rules/shadcn-ui.md. Browser-verified per .claude/rules/ui-design.md.

[w7-mgfcode]

Shipped in v0.2.15 via PR #211 — light/dark theme redesign for cohesion and accessibility.