CL-5781: Theme system — light/dark toggle + preset themes

[TheGreatAxios]

Summary

• Mounts library ThemeProvider / ThemeToggle from @corbits/react-ui (pin bd5057b) at the app root and rail identity dock
• Per-user theme storage key when signed in; anonymous key when signed out
• Tokenizes package colors (chat-ui agent accent, settings-ui destructive, bench-ui shadows, artifact-ui kind chart tokens)
• Shell/app tests wrap ThemeProvider; Tailwind utility guards cover chart-token kind colors

Theme logic lives in corbitsdev/react-ui (PRs #12 theme system, #13 packaging). Workbench stays a thin consumer.

Closes CL-5781.

Test plan

• bun run typecheck (web + UI packages)
• bun test ./test in apps/web (shell docks asserts theme control)
• bun test artifact-ui / chat-ui / bench-ui / settings-ui
• bun run scripts/checks/web-tailwind-utilities.ts
• CI green
• Manual: toggle light/dark and warm/cool presets in the rail