chore: add shadcn configuration and btn component

[peppescg]

[Copilot]

Pull Request Overview

This PR adds shadcn/ui configuration and implements a Button component with variant support. It sets up the necessary infrastructure for using shadcn/ui components including utility functions for className merging, CSS variables for theming with dark mode support, and the required dependencies.

• Added shadcn/ui configuration with path aliases and styling preferences
• Implemented a Button component with multiple variants (default, destructive, outline, secondary, ghost, link) and sizes
• Updated CSS variables from hex colors to HSL format for better theming support
• Added required dependencies: class-variance-authority, clsx, and tailwind-merge
• Downgraded better-auth from 1.4.0-beta.21 to 1.4.0-beta.20

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/lib/utils.ts	Adds cn utility function for merging Tailwind classes using clsx and tailwind-merge
src/components/ui/button.tsx	Implements Button component with variants using class-variance-authority and React forwardRef
src/app/globals.css	Converts color system from hex to HSL CSS variables, adds dark mode theme, and expands color palette for shadcn/ui
components.json	Adds shadcn/ui configuration file with path aliases and style settings
package.json	Adds shadcn/ui dependencies (class-variance-authority, clsx, tailwind-merge) and downgrades better-auth
pnpm-lock.yaml	Updates lockfile to reflect dependency changes

Files not reviewed (1)

• pnpm-lock.yaml: Language not supported

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