Enhance external login and proxy compatibility

- Updated `ExternalLoginPicker.razor` to add Google and Microsoft login buttons with SVG icons and improved styling.
- Added new styles in `ExternalLoginPicker.razor.css` for layout and accessibility.
- Configured `ForwardedHeadersOptions` and enabled `UseForwardedHeaders` middleware in `Program.cs` for reverse proxy support.
- Added `AddCascadingAuthenticationState` for better authentication state management.
- Included `microsoft-identity-association.json` for Microsoft app association.
- Added `google-signin.svg` and `microsoft-signin.svg` assets for login buttons.

src/BirthList.Web/Components/Account/Shared/ExternalLoginPicker.razor.css

@@ -0,0 +1,29 @@
+.external-login-form {
+    display: block;
+}
+
+.external-login-actions {
+    display: flex;
+    flex-direction: column;
+    gap: 0.75rem;
+    align-items: flex-start;
+}
+
+.external-login-btn {
+    padding: 0;
+    border: none;
+    background: transparent;
+    line-height: 0;
+    border-radius: 4px;
+}
+
+.external-login-btn img {
+    height: 40px;
+    width: auto;
+    display: block;
+}
+
+.external-login-btn:focus-visible {
+    outline: 2px solid #0d6efd;
+    outline-offset: 2px;
+}