feat(admin): add light theme appearance preferences

[DavidBabinec]

Summary

• Adds persistent user-level admin appearance preferences for theme and UI text scale, alongside the existing density setting.
• Adds light theme token overrides for admin chrome, including readable identity accents, overlay-based neutral controls, and clearer body/surface separation.
• Keeps editor layout roots and portaled UI surfaces in sync through data-editor-theme, data-editor-density, and data-editor-text-scale attributes.
• Adds regression coverage for appearance preference persistence, light-theme accent contrast, light body/surface contrast, and neutral control backgrounds.

Why

Issue #138 asks for a light version of Instatic and persistent user appearance settings. The previous light-theme pass also exposed low contrast between pale surfaces, rail tint colors, badges, and active controls. This PR moves those differences into global tokens and shared primitives so the behavior is consistent across admin workspaces.

Closes #138.

User / Developer Impact

• Users can switch the admin UI to light mode and adjust UI text scale from preferences.
• Light mode keeps dashboard, editor sidebars, segmented controls, buttons, rails, badges, and inspector groups readable on white surfaces.
• New tests lock down token contrast and prevent shared chrome from drifting back to surface-only states.

Verification

• npx react-doctor@latest --verbose --scope changed --base origin/main → no issues found
• bun test → 5,919 pass, 0 fail
• bun run build → exit 0
• bun run lint → exit 0
• git diff --check → exit 0
• Browser QA on http://localhost:5173/admin/dashboard and http://localhost:5173/admin/site for light-theme contrast fixes