fix: resolve Tailwind v4 CSS layer conflict with CopilotKit

[Abubakar-01]

Summary

Fixes build error caused by CopilotKit v2 CSS using Tailwind v4 @layer syntax in a project still configured for Tailwind v3.

Changes

• Upgraded Next.js to 15.2.4 to support Tailwind v4's new CSS pipeline
• Installed @tailwindcss/postcss and updated PostCSS config
• Removed outdated Tailwind v3 directives from globals.css

Result

Build now completes without @layer base CSS syntax errors.

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