refactor(BlockEditor): replace emoji icons with react icon components and add free color picker support

- update blockRegistry to accept ReactNode icons instead of emoji strings
- replace emoji icons in all built-in block types with icon components from shared icons
- add `isHexColor` and `collectUsedColors` helpers to inline/types.js
- extend `color` and `highlight` marks to accept hex color strings in addition to palette keys
- pass `usedColors` (collected from document) to InlineToolbar
- update InlineToolbar color popover to show used colors and a free color input
- add new icons to shared icons index
- update README to reflect icon, color, and toolbar popover changes

src/shared/components/BlockEditor/blockRegistry.js

@@ -6,7 +6,7 @@
 // {
 //   type: string,                  // id unique (ex: 'paragraph', 'my_custom')
 //   label: string,                 // libellé affiché dans le slash menu
-//   icon: string,                  // glyphe court (emoji ou caractère)
+//   icon: ReactNode,               // élément React rendu dans le slash menu (typiquement une icône de `@zen/core/shared/icons`)
 //   keywords: string[],            // termes de recherche pour le slash menu
 //   shortcut?: string,             // préfixe markdown qui convertit (ex: '# ', '- ')
 //   shortcutTransform?: (block, match) => block,  // optionnel : transforme un bloc existant