fix: add controls for toolbar overflow

[fredrikblau]

Description

On narrow windows the editor toolbar exposes items through horizontal scrolling, but users with a mouse without horizontal scrolling have no obvious way to reach them.

Add accessible left and right controls whenever the toolbar overflows. Each control scrolls the toolbar by 75% of its visible width, while the existing native scrolling remains available.

Related Issue

• Fixes Tiptap: horizontal overflow in toolbar not usable #3183

How Has This Been Tested?

• test environment: Node.js 26.7.0, pnpm 11.24.0
• test case 1: pnpm test:unit --run packages/web-pkg/tests/unit/editor/components/TextEditorToolbar.spec.ts
• test case 2: pnpm check:types
• test case 3: pnpm lint
• test case 4: pnpm format:check

Types of changes

• Bugfix
• Enhancement (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt (improving code quality without changing functionality)
• Tests (adding or updating tests)
• Documentation (updates to the documentation, readme, or changelog)
• Maintenance (updates to the build process or auxiliary tools and libraries)

[AlexAndBear]

That's a great start, thank you 🙏

Could you try to improve the visual appearance of this feature ?

• The scroll bar is now showing up while using the < > buttons and is quite near to the toolbar, therefore the UI looks very cluttered.

• The < > buttons do overflow the other toolbar items making them hard to see and operate. Could think of multiple ways to solve it e.G giving the toolbar a padding left and right when the < > buttons are displayed or giving them a background with a drop shadow to either right or left.
  Whatever works best 👍

[fredrikblau]

Addressed the toolbar feedback: hid the native scrollbar, added side padding, and gave the scroll controls a surface/shadow so they remain usable over toolbar items. Targeted test, Prettier, and ESLint pass.