Upgrade react 19

[lebalz]

reopens #81 (which was merged/reverted by accident)

• use React.ReactNode instead of JSX.Element
• updare various packages
• ensure useRef is initialized with a value

Still blocked by: excalidraw/excalidraw#8923
Fixed Excalidraw import issues through configuration of fallbacks:

// docusaurus.config.ts#webpack
...
plugins: [
() => {
      return {
          name: 'excalidraw-config',
          configureWebpack(config, isServer, {currentBundler}) {
            return {
              module: {
                rules: [
                  {
                    test: /\.excalidraw$/,
                    type: 'json',
                  },
                  {
                    test: /\.excalidrawlib$/,
                    type: 'json',
                  }
                ],
              },
              resolve: {
                fallback: {
                  'roughjs/bin/math': path.resolve(__dirname, './node_modules/roughjs/bin/math.js'),
                  'roughjs/bin/rough': path.resolve(__dirname, './node_modules/roughjs/bin/rough.js'),
                  'roughjs/bin/generator': path.resolve(__dirname, './node_modules/roughjs/bin/generator.js')
                }
              },
              plugins: [
                new currentBundler.instance.DefinePlugin({
                  'process.env.IS_PREACT': JSON.stringify('false')
                }),
              ]
            }
          }
      }
    }
]

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[SilasBerger]

Tested in my own deployment, works without issues.