Add support for React 19

[couvq]

Description

Currently Flourish does not support React 19. Here is the terminal output while running npm i flourish-ui:

couvq@Quentins-MacBook-Air kanban % npm i flourish-ui
npm error code ERESOLVE
npm error ERESOLVE unable to resolve dependency tree
npm error
npm error While resolving: kanban@0.0.0
npm error Found: react@19.1.1
npm error node_modules/react
npm error   react@"^19.1.0" from the root project
npm error
npm error Could not resolve dependency:
npm error peer react@"^18.2.0" from flourish-ui@1.0.17
npm error node_modules/flourish-ui
npm error   flourish-ui@"*" from the root project
npm error
npm error Fix the upstream dependency conflict, or retry
npm error this command with --force or --legacy-peer-deps
npm error to accept an incorrect (and potentially broken) dependency resolution.
npm error
npm error
npm error For a full report see:
npm error /Users/couvq/.npm/_logs/2025-08-02T15_43_41_174Z-eresolve-report.txt
npm error A complete log of this run can be found in: /Users/couvq/.npm/_logs/2025-08-02T15_43_41_174Z-debug-0.log

Ideally flourish works with React 19, while still supporting react 18.

References

• React 19 upgrade guide
• MUI codebase as an example

Acceptance criteria

• flourish-ui can be installed and used in apps using React 19
• flourish-ui can still be installed and used in apps using React 18