[feat] Add Special Characters Button

[Kniggishood]

Adds a new BtnSpecialCharacters component that opens a modal with Greek letters, math symbols, and arrows.

Why

For our Flowcharts initiative, we currently use a custom editor. Users frequently need to insert special characters in rich text editors. This provides an intuitive way to access common symbols.

How

• New BtnSpecialCharacters component using existing createButton factory
• Responsive grid modal with 36 commonly-used characters
• Uses existing CSS patterns (rsw-modal, rsw-btn)
• Semantic HTML with proper accessibility attributes

Testing

• Added to default toolbar in DefaultEditor
• E2E tests updated to handle new UI
• Screenshot tests updated

Breaking Changes

None - this is purely additive

[megahertz]

Thank you for the PR. I'm not sure if it should be included in the main library until I make it truly tree-shakable. Now the priority is the package size. I'm thinking about adding subpackages. I can't promise I'll make this decision quickly, but I would prefer to save your work anyway.

[Kniggishood]

Thank you for the PR. I'm not sure if it should be included in the main library until I make it truly tree-shakable. Now the priority is the package size. I'm thinking about adding subpackages. I can't promise I'll make this decision quickly, but I would prefer to save your work anyway.

Hey @megahertz – sorry for the long wait and thanks for the feedback! I'll close the PR for now then. Gladly keep me posted.

[megahertz]

Thanks @Kniggishood . I want to keep this PR open as a reminder for myself. I have been moving to a new country for the last few months, which is why I cannot contribute as much. I'm still considering creating a monorepo and creating a subpackage for this button.