fix(frontend): make theme toggle apply light mode

[deepakvarma1536]

Summary

• Rebased the light/dark theme fix onto the latest main after PR fix light and dark theme toggle #1208 was closed as no longer merging cleanly.
• Enables Tailwind class-based dark mode and adds a light-mode CSS variable palette.
• Updates dashboard stat text to use semantic theme-aware colors so values remain readable in light mode.
• Adds tests covering root theme classes and localStorage persistence for the theme toggle.

Closes #1089
Supersedes #1208

Verification

• cd frontend && npm run typecheck
• cd frontend && npm run build -- --logLevel error
• cd frontend && npm run test attempted locally, but local Node is v18.19.1 while the repo recommends Node 20+; Vitest/jsdom fails before running tests with ERR_REQUIRE_ESM from html-encoding-sniffer requiring @exodus/bytes/encoding-lite.js.

Notes

• This is intentionally frontend-only and keeps the diff focused to the theme toggle/light-mode issue.
• No backend/API changes or migrations.