feat: use shadow dom

[njesenberger]

Migrate toolbar to Shadow DOM for style isolation

This PR moves the toolbar into a Shadow DOM host, preventing CSS from leaking between the toolbar and the host page in either direction.

What changed

Shadow DOM encapsulation
The toolbar's React tree is now rendered inside a shadow root via a new ShadowRoot component. All stylesheets are collected as raw CSS strings and injected as a single <style> tag inside the shadow root, replacing the previous approach of scattering individual <style> tags across document.head. A new reset.scss provides baseline normalization scoped to :host so host-page resets can't bleed in.

Build pipeline refactor
The scssModulesPlugin in tsup.config.ts has been simplified. Each SCSS file previously generated runtime code that appended style elements to document.head — now CSS modules export a named css string alongside the default class-name map, and plain .scss files export only a css string. The component concatenates these into one <style> tag rendered inside the shadow root. scss.d.ts is updated to match, and esbuild is now an explicit dev dependency rather than a transitive one.

Color token CSS
The injectAgentationColorTokens function that imperatively wrote to document.head at module load time has been replaced with a static CSS string using :host selectors, injected alongside the other styles inside the shadow root.

CSS fixes for Shadow DOM compatibility
Two :has() selectors that don't work reliably across shadow boundaries have been replaced with explicit data-checked attribute checks in Checkbox and Switch. Both components now set data-checked on their wrapper div and use .container[data-checked] in their stylesheets instead.

Bug fixes

• The drag preview element is appended to portalWrapperRef.current (inside the shadow DOM) rather than document.body, so it picks up shadow-scoped styles correctly
• RearrangeOverlay now queries its own shadow root for [data-rearrange-section] elements instead of document, preventing potential mismatches
• Cursor override styles no longer need a :not([data-agentation-root] *) exclusion since the toolbar is naturally isolated inside its shadow root
• webkit-user-select: none added throughout for Safari compatibility

Cleanup
Removed dead CSS from styles.module.scss: unused .canvasPurpose* blocks and .customCheckbox, .toggleLabel, .toggleSwitch, .toggleSlider, which are superseded by the Checkbox and Switch components.

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