Unify all formatting buttons to use consistent three-layer architecture

- Remove .insertr-default-style special styling (blue background, border, dot indicator)
- Make ALL buttons use .insertr-style-preview with unified appearance
- Maintain authentic style previews in isolated .insertr-style-sample layer
- Bold buttons show bold text, .brand buttons show green uppercase text, etc.
- Eliminate visual inconsistency between semantic and detected style buttons
- Simplify CSS by removing ~50 lines of duplicate button styling
- Provide consistent professional toolbar appearance across all formatting options

CLASSES.md

@@ -256,8 +256,8 @@ The `.insertr-add` class transforms a container into a dynamic collection where
 - Styling: All classes and structure preserved
 
 ### User Interface
-- **Add Button**: Floating "+" button. 
-- **Remove Controls**: Delete option for each item (with confirmation)
+- **Add Button**: Floating "+" button. Should be in the top right of the .insertr-add container.
+- **Remove Controls**: Delete option for each item (with confirmation). Should be in the top tight corner of the item.
 - **Reorder Capability**: Drag-and-drop or up/down controls
 - **Duplication**: New items inherit the exact structure and styling