feat: Enable theme inheritance and remove duplicate files

Connect Theme#view_paths to controller for proper parent-child inheritance.
Child themes now automatically fallback to parent theme views when a file
doesn't exist locally.

Changes:
- Update set_theme_path in ApplicationController to use Theme model
- Remove 19 identical files from brisbane and biarritz themes
- Add BEM implementation plan documentation

This reduces maintenance burden for future CSS/styling updates since
changes to the default theme will automatically propagate to child themes.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: etewiah

app/controllers/pwb/application_controller.rb

@@ -38,8 +38,17 @@ def set_theme_path
         end
       end
       theme_name = theme_name.present? ? theme_name : "default"
-      # Use Rails.root for theme paths
-      prepend_view_path "#{Rails.root}/app/themes/#{theme_name}/views/"
+
+      # Use Theme model's view_paths for proper inheritance support
+      # This allows child themes to automatically inherit views from parent themes
+      theme = Theme.find_by(name: theme_name) || Theme.find_by(name: "default")
+      if theme
+        # Prepend in reverse order so theme-specific paths have highest priority
+        theme.view_paths.reverse_each { |path| prepend_view_path path }
+      else
+        # Fallback to hardcoded path if theme not found
+        prepend_view_path "#{Rails.root}/app/themes/#{theme_name}/views/"
+      end
 
       self.class.layout "layouts/pwb/application"
     end