Harden OIDC, add SVG sanitization, improve form UX and security defaults

Remove PKCE plain method support (S256 only), enforce openid scope requirement,
filter to supported scopes, strip reserved claims from custom claims as
defense-in-depth, sanitize SVG icons with Loofah, add global input padding,
switch session cookies to SameSite=Lax, use Session.active scope, and remove
unsafe-eval from CSP.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/controllers/admin/users_controller.rb

@@ -122,15 +122,14 @@ module Admin
     end
 
     def user_params
-      # Base attributes that all admins can modify
-      base_params = params.require(:user).permit(:email_address, :username, :name, :password, :status, :totp_required, :custom_claims)
+      permitted = [:email_address, :username, :name, :password, :status, :totp_required, :custom_claims]
 
       # Only allow modifying admin status when editing other users (prevent self-demotion)
       if params[:id] != Current.session.user.id.to_s
-        base_params[:admin] = params[:user][:admin] if params[:user][:admin].present?
+        permitted << :admin
       end
 
-      base_params
+      params.require(:user).permit(*permitted)
     end
   end
 end