Upgrade React to `19.x`

[lemonpole]

In #466 a dropdown was added which makes use of new HTML components for which the current version of React does not have types for.

React must be upgraded as well as React Markdown.

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Important

The @g-loot/react-tournament-brackets repo is no longer maintained and was migrated to https://github.com/Shenato/react-tournament-brackets.

Looks like @g-loot/react-tournament-brackets@1.0.31-rc requires React 18.x.

npm error code ERESOLVE
npm error ERESOLVE could not resolve
npm error
npm error While resolving: @g-loot/react-tournament-brackets@1.0.31-rc
npm error Found: react@19.1.1
npm error node_modules/react
npm error   react@"19.1.1" from the root project
npm error   peer react@">=16.8.0" from @emotion/react@11.14.0
npm error   node_modules/@emotion/react
npm error     @emotion/react@"^11.8.1" from react-select@5.10.2
npm error     node_modules/react-select
npm error       react-select@"5.10.2" from the root project
npm error   11 more (@emotion/use-insertion-effect-with-fallbacks, ...)
npm error
npm error Could not resolve dependency:
npm error peer react@"^18.1.0" from @g-loot/react-tournament-brackets@1.0.31-rc
npm error node_modules/@g-loot/react-tournament-brackets
npm error   @g-loot/react-tournament-brackets@"1.0.31-rc" from the root project
npm error
npm error Conflicting peer dependency: react@18.3.1
npm error node_modules/react
npm error   peer react@"^18.1.0" from @g-loot/react-tournament-brackets@1.0.31-rc
npm error   node_modules/@g-loot/react-tournament-brackets
npm error     @g-loot/react-tournament-brackets@"1.0.31-rc" 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.