feat(branding,auth): Update app description and improve authentication flow

- Update manifest.json and layout metadata with comprehensive Synapsis description emphasizing decentralized infrastructure and global signal layer concept
- Expand login page default node description to match updated branding messaging
- Replace Next.js router navigation with hard window.location.href redirect to ensure authentication cookies are properly picked up after login/import
- Remove unused router and ShieldAlert imports from login page
- Add accent color luminance detection to ToastContext for dynamic text color contrast
- Implement intelligent text color selection in success toasts based on accent color brightness to ensure readability
- These changes improve authentication reliability and ensure consistent branding messaging across the application

public/manifest.json

@@ -1,7 +1,7 @@
 {
     "name": "Synapsis",
     "short_name": "Synapsis",
-    "description": "Federated social network infrastructure",
+    "description": "Synapsis is designed to function like a global signal layer rather than a culture-bound platform. Anyone can run their own node and still participate in a shared, interconnected network.",
     "start_url": "/",
     "display": "standalone",
     "background_color": "#0a0a0a",