Replace "FastNext" branding with "PragmaStack" across the project

- Updated all references, metadata, and templates to reflect the new branding, including layout files, components, and documentation.
- Replaced hardcoded color tokens like `green-600` with semantic tokens (`success`, `warning`, etc.) for improved design consistency.
- Enhanced `globals.css` with new color tokens for success, warning, and destructive states using the OKLCH color model.
- Added comprehensive branding guidelines and updated the design system documentation to align with the new identity.
- Updated tests and mocks to reflect the branding changes and ensured all visual/verbal references match "PragmaStack".
- Added new `branding/README.md` and `branding` docs for mission, values, and visual identity definition.

frontend/src/components/layout/Header.tsx

@@ -83,7 +83,7 @@ export function Header() {
         {/* Logo */}
         <div className="flex items-center space-x-8">
           <Link href="/" className="flex items-center space-x-2">
-            <span className="text-xl font-bold text-foreground">FastNext</span>
+            <span className="text-xl font-bold text-foreground">PragmaStack</span>
           </Link>
 
           {/* Navigation Links */}