docs: Document npm update and Tailwind v4 PostCSS fixes

- Add Build System Updates section documenting npm version update
- Document Tailwind CSS v4 PostCSS configuration changes
- Update Last Updated timestamp

DEPENDENCY_UPDATES.md

@@ -1,5 +1,27 @@
 # Dependency Updates Summary
 Generated: 2026-02-19
+Last Updated: 2026-02-20
+
+---
+
+## Build System Updates (2026-02-20)
+
+### npm Version Update:
+- **Backend Dockerfile**: Updated to install npm@latest globally during build
+  - Previous: npm 10.8.2 (bundled with Node 20)
+  - Updated: npm 11.10.1
+  - Change: Added `npm install -g npm@latest` after bun installation
+  - Benefit: Latest npm features, bug fixes, and security updates
+
+### Tailwind CSS v4 Compatibility Fixes:
+- **Frontend PostCSS Configuration**: Updated to use new Tailwind v4 plugin
+  - Previous: `tailwindcss: {}` in postcss.config.js
+  - Updated: `'@tailwindcss/postcss': {}` in postcss.config.js
+  - Added: `@tailwindcss/postcss: ^4.0.0` to devDependencies
+  - Fixes: Build error "The PostCSS plugin has moved to a separate package"
+  - Compatibility: Tailwind CSS v4.2.0
+
+---
 
 ## Backend Dependencies Updates
 
@@ -90,6 +112,7 @@ Generated: 2026-02-19
 
 ### High Priority:
 1. **tailwindcss v4**: This is a major update with significant changes. The project's configuration files (tailwind.config.js, postcss.config.js) may need updates. Note: Tailwind v4 is designed to be backward compatible with v3 configs in most cases.
+   - **FIXED (2026-02-20)**: Updated postcss.config.js to use `@tailwindcss/postcss` plugin instead of `tailwindcss` directly. Added `@tailwindcss/postcss: ^4.0.0` to devDependencies.
 
 ### Medium Priority:
 2. **zod v4**: Check if validation schemas need updates.