Add Shadcn Theme plugin submission

Introduces the Filament Shadcn Theme plugin to the directory - a unique theme implementation that brings Shadcn UI's adaptive color system to Filament.

Key Features:
- Adaptive colors that change between light and dark modes
- 8 official Shadcn UI themes (Default, Red, Rose, Orange, Green, Blue, Yellow, Violet)
- Custom adaptive color support for brand-specific requirements
- Perfect contrast in both light and dark modes

Plugin Details:
- Package: openplain/filament-shadcn-theme
- Repository: https://github.com/openplain/filament-shadcn-theme
- Requirements: PHP 8.1+, Filament 4.0+
- Category: Theme
- Dark theme support: Yes
- Translations: No

This is the only Filament theme that implements Shadcn UI's signature design pattern where colors invert between modes (dark buttons in light mode, light buttons in dark mode) rather than keeping the same colors across both themes.

Author: mashema

content/plugins/images/openplain-shadcn-theme.jpg

@@ -0,0 +1,13 @@
+---
+name: Shadcn Theme
+slug: openplain-shadcn-theme
+author_slug: openplain
+categories: [theme]
+description: Shadcn UI theme for Filament with adaptive colors that change between light and dark modes - 8 beautiful official themes included
+docs_url: https://raw.githubusercontent.com/openplain/filament-shadcn-theme/refs/heads/main/README.md
+github_repository: openplain/filament-shadcn-theme
+has_dark_theme: true
+has_translations: false
+versions: [4]
+publish_date: 2025-12-08
+---