feat(custom-apps): shadcn-ready app templates [LSDK-331]

[Hari Harish (harisaiharish)]

• Adds a pre-made components.json and @/* path aliases (tsconfig + vite) so scaffolded apps work with the shadcn CLI directly — shadcn init is never needed, and would otherwise install a competing preset that breaks the build.
• Pre-wires the design-system theme import and Tailwind preset against overwritable placeholders, so a bare scaffold still builds and the first shadcn add --overwrite swaps in the real files.
• Documents the registry in AGENTS.md, including the required --overwrite flag. Templates otherwise only gain shadcn as a devDependency; replacing the hand-rolled components with real DS ones, and pulling them at init, lands separately.