Add a theme system and light/dark mode to the shell

[priethor]

Goal

Phase 1 of the Cortext editing theme: declare the token contract, ship the light/dark toggle, and refactor existing shell styles to consume tokens.

Scope

• Declare the token contract: color, typography, spacing, borders, and shadow tokens, light and dark.
• Refactor shell styles to consume tokens.
• Light / dark / auto toggle in the shell, persisted per user.
• Stamp the chosen mode before React mounts so dark users don't get a light flash on reload.
• Drop the frontend token overlay and the Cortext header/footer block patterns; Phase 1 doesn't put any Cortext styling on the public side.
• Document the token contract in docs/theming.md.

Out of scope

• Extending the theme into the canvas iframe.
• The Site Preview toggle.
• Accent picker, font picker, density.

Acceptance criteria

• Light, dark, and auto modes work in the shell and persist per user across reloads.
• The shell renders identically in light mode before and after the token refactor (no visual regression).
• The plugin emits no Cortext CSS, patterns, or tokens on public pages.
• docs/theming.md documents the token contract.