fix(build): Fix build errors in backend and frontend

Backend:
- Add missing path mapping for @freundebuch/shared/index.js in tsconfig.build.json

Frontend:
- Migrate to TailwindCSS v4 configuration model
- Add @tailwindcss/postcss package for PostCSS integration
- Update app.css to use @import "tailwindcss" and @theme directive
- Move custom colors and fonts to CSS-based @theme configuration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: enko

apps/backend/tsconfig.build.json

@@ -6,7 +6,8 @@
     "module": "NodeNext",
     "moduleResolution": "NodeNext",
     "paths": {
-      "@freundebuch/shared": ["../../packages/shared/src/index.ts"]
+      "@freundebuch/shared": ["../../packages/shared/src/index.ts"],
+      "@freundebuch/shared/index.js": ["../../packages/shared/src/index.ts"]
     }
   },
   "include": ["src/**/*"],

apps/frontend/package.json

@@ -23,6 +23,7 @@
     "@sveltejs/adapter-auto": "7.0.0",
     "@sveltejs/kit": "2.49.2",
     "@sveltejs/vite-plugin-svelte": "6.2.1",
+    "@tailwindcss/postcss": "4.1.18",
     "@testing-library/svelte": "5.3.0",
     "autoprefixer": "10.4.23",
     "jsdom": "27.3.0",

apps/frontend/postcss.config.js

@@ -1,6 +1,6 @@
 export default {
   plugins: {
-    tailwindcss: {},
+    '@tailwindcss/postcss': {},
     autoprefixer: {},
   },
 };

apps/frontend/src/app.css

@@ -1,6 +1,17 @@
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
+@import "tailwindcss";
+
+@theme {
+  /* Custom colors */
+  --color-forest: #2d5016;
+  --color-forest-light: #3a6b1e;
+  --color-forest-dark: #1f3810;
+  --color-sage: #8b9d83;
+  --color-amber-warm: #d4a574;
+
+  /* Custom fonts */
+  --font-heading: "Yanone Kaffeesatz", sans-serif;
+  --font-body: Merriweather, serif;
+}
 
 @layer base {
   body {