Enhance IndexPage UI and navigation functionality

- Injected `NavigationManager` into `IndexPage.razor` to enable navigation.
- Improved layout alignment with additional classes in `<div>`.
- Added a new button for navigation to `/sender` with icons and text.
- Introduced a feature badge for "PDF-Export" with an SVG icon.
- Updated `.home-btn-primary` styles in `app.css`:
  - Ensured text color is always white with `!important`.
  - Made text bolder by increasing `font-weight` to `700`.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/css/app.css

@@ -308,8 +308,8 @@ article {
 .home-btn-primary {
     background: linear-gradient(135deg, #2c3e50 0%, #3498db 100%);
     border: none;
-    color: #fff;
-    font-weight: 500;
+    color: #fff !important;
+    font-weight: 700;
     border-radius: 8px;
     transition: filter 0.15s, box-shadow 0.15s;
     box-shadow: 0 2px 10px rgba(44,62,80,0.30);