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

src/app/layout.tsx

@@ -15,7 +15,7 @@ const sairaCondensed = Saira_Condensed({
 
 export const metadata: Metadata = {
   title: "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, with global identity, clean terminology, and a modern interface that feels current rather than experimental.",
   manifest: "/manifest.json",
   icons: {
     icon: "/favicon.png",