feat(admin): add theme preferences and refine dark mode styling

[coding-hui]

Summary

• Add a real admin theme switcher with light, dark, and system modes, defaulting to system preference.
• Remove the developer SettingDrawer from the runtime UI and disable MFSU to avoid stale module federation runtime failures during local dev.
• Normalize admin status tags, page descriptions, buttons, links, and account settings styling across light and dark themes.
• Add focused tests for theme preference normalization and subscriptions.

User Impact

Admins can switch themes from the top header without seeing development-only controls. Dark mode now has readable primary/default buttons, table actions, status tags, page descriptions, and account settings surfaces while preserving the light theme appearance.

Validation

• pnpm --dir apps/admin biome
• gofmt -w $(rg --files -g '*.go')
• pnpm --dir apps/admin lint
• pnpm --dir apps/admin jest --runInBand
• pnpm --dir apps/admin build
• go test ./...
• Chrome regression checks for light, dark, and system theme modes on admin pages

Note: admin build still reports the existing Browserslist/caniuse-lite freshness warning; build completes successfully.