Add new portable UI app shell (apps/ui)

[youknowriad]

Related issues

• Related to the new UI initiative

How AI was used in this PR

The app shell was pair-programmed with Claude Code. All code has been reviewed and tested manually.

Proposed Changes

Introduces apps/ui, a new portable UI layer that runs as both an Electron renderer and a standalone web app:

• Connector pattern: Injectable data adapter (Connector interface) with two implementations — IPC for Electron (window.ipcApi) and REST for web (Telex API at telex.automattic.ai + WordPress.com OAuth)
• TanStack Router (code-based, following wp-calypso patterns) with auth-guarded routes and layout routes for dashboard (sidebar) and onboarding (full-screen)
• TanStack Query with localStorage persistence (24h max age), mirroring the wp-calypso api-core setup
• WordPress Design System: @wordpress/ui, @wordpress/theme (ThemeProvider via private APIs), @wordpress/icons
• Component structure: Folder-per-component with CSS Modules, dark/light mode via prefers-color-scheme
• Dual dev modes: npm -w @studio/ui run dev:web (port 5200) and npm run start:new-ui (full Electron app)
• Default color scheme: Changed from 'light' to 'system' so prefers-color-scheme works correctly

Testing Instructions

• Run npm -w @studio/ui run dev:web and verify the login page appears at localhost:5200
• Log in with WordPress.com OAuth and verify redirect back to dashboard
• Run npm run start:new-ui to verify the Electron app launches with the new UI
• Toggle system dark/light mode and verify the UI follows

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?