feat(ui): scaffold package and shadcn routing

[kattsushi]

Closes #1

PR Type

• Bug fix
• New feature
• Documentation only
• Code refactoring
• Maintenance/tooling
• Breaking change

Summary

• Scaffold @os-copilot/ui as a Solid + TypeScript Nx/Vite library package.
• Configure app/package shadcn monorepo routing and verify Zaidan button installs into packages/ui.
• Add PRD, Nx MCP config, catalog-backed UI dependencies, and smoke tests for root/subpath/source-routed exports.

Chain Context

master
└── PR 0 tracker: feat/ui-package (#2)
    └── 📍 PR 1: feat/ui-package-scaffold-routing
        └── PR 2: ui-package-zaidan-storybook-css
            └── PR 3: ui-package-custom-verification

PR	Scope	Base
#2	Draft/no-merge tracker branch	master
This PR	Scaffold package + shadcn routing smoke	feat/ui-package
PR 2	Zaidan baseline/components + Storybook/CSS	this branch
PR 3	Custom components + SSR/SPA/tree-shaking/visual verification	PR 2 branch

Changes

File	Change
.mcp.json	Add project-local Nx MCP config using pnpm exec nx mcp.
docs/prd-ui-package.md	Add PRD for the mobile-first Solid UI package.
apps/web/components.json	Configure shadcn app aliases so UI installs route to @os-copilot/ui/components/ui.
apps/web/package.json	Add workspace dependency on @os-copilot/ui and package-local #... imports.
packages/ui/*	Scaffold UI package, Nx targets, Vite/Vitest/TS configs, Zaidan button, CSS, and export smoke test.
pnpm-workspace.yaml	Add UI dependency versions to catalog.
pnpm-lock.yaml	Update lockfile for UI package dependencies.

Test Plan

• RED evidence before scaffold: pnpm nx show project ui failed with Could not find project ui
• RED evidence before scaffold: pnpm nx typecheck ui failed with Cannot find project 'ui'
• RED evidence before scaffold: pnpm nx build ui failed with Cannot find project 'ui'
• shadcn routing dry-run from apps/web first exposed wrong path, then passed after alias fix
• cd apps/web && pnpm dlx shadcn@latest add @zaidan/button --overwrite --yes
• pnpm exec dprint check
• pnpm exec oxlint
• pnpm nx typecheck ui
• pnpm nx build ui
• pnpm nx test ui --run
• pnpm nx typecheck web

Contributor Checklist

• Linked an approved issue
• Added exactly one type:* label
• Ran shellcheck on modified scripts (not applicable; no scripts changed)
• Skills tested in at least one agent
• Docs updated if behavior changed
• Conventional commit format
• No Co-Authored-By trailers