Rewrite widgets stack with shadcn and Tailwind v4 scaffold

[emmapowers]

This branch removes the legacy widget/design-system stack and replaces it with a new shadcn/Base UI/Tailwind v4 foundation. It keeps the existing runtime theme-mode plumbing, adds a generic Tailwind bundling step, introduces a minimal shadcn-backed smoke widget, and moves the shadcn/Bun tooling package out of src/ so local node_modules cannot interfere with Trellis bundling.

Main changes:

• remove the legacy widget/design-system surface and establish new trellis.theme and trellis.widgets package boundaries
• add shadcn/Base UI/Tailwind v4 frontend scaffolding, a generic Tailwind build step, and a smoke Button widget plus example app
• switch repo-managed JS tooling from npm to Bun and add pixi tasks for widget tooling workflows
• update runtime theme application to use shadcn-style .dark root classes and keep server/browser/desktop paths aligned
• add focused Python and JS coverage for dependency alignment, Tailwind bundling, theme application, and the smoke widget path

Misc:

• the shadcn tooling package now lives in frontend/trellis-widgets so checked-in tooling node_modules cannot affect application builds
• desktop smoke bundling and runtime were revalidated after removing the temporary React alias workaround

Testing:

• pixi run widgets-typecheck
• pixi run trellis -r examples/shadcn_smoke bundle --platform server --force-build
• pixi run trellis -r examples/shadcn_smoke bundle --platform desktop --force-build
• pixi run trellis -r examples/shadcn_smoke run --desktop
• pixi run ci

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