refactor: remove `theme` `defaultProp`

[ze-flo]

Description

This initiates the process of removing defaultProps to avoid deprecation warnings, focusing first on theme. Since v9 depends entirely on ThemeProvider and its theme for dark mode, rendering a component without wrapping it in ThemeProvider should result in failure.

Detail

• Removes eslint-plugin-garden-local
• ThemeProvider: sets the theme prop's default to DEFAULT_THEME
• Removes theme defaultProp in all components
• Removes unnecessary test

Checklist

• [ ] :ok_hand: design updates will be Garden Designer approved (add the designer as a reviewer)
• [ ] :globe_with_meridians: demo is up-to-date (npm start)
• [ ] :arrow_left: renders as expected with reversed (RTL) direction
• [ ] :black_circle: renders as expected in dark mode
• [ ] :metal: renders as expected with Bedrock CSS (?bedrock)
• [ ] :guardsman: includes new unit tests. Maintain existing coverage (always >= 96%)
• [ ] :wheelchair: tested for WCAG 2.1 AA accessibility compliance
• [ ] :memo: tested in Chrome, Firefox, Safari, and Edge

[ze-flo]

Here's a table comparing popular React component libraries written with styled-components or emotion.js. All support theming and light / dark mode. The table highlights whether each library can render components without first nesting them inside a ThemeProvider.

Library	Works without ThemeProvider	Notes	Docs	Sandbox
@mui/material	✅	DefaultProps injected via React's context API with defaultPropsProvider	https://mui.com/material-ui/getting-started/usage/	https://codesandbox.io/p/sandbox/jgtp7x
@mantine/core	❌		https://mantine.dev/guides/vite/#setup
@primer/react	❌		https://primer.style/guides/react
@chakra-ui/react	❌		https://v2.chakra-ui.com/getting-started
@adobe/react-spectrum	❌	Components requiring theme render null if its missing. Others render unstyled.	https://react-spectrum.adobe.com/react-spectrum/getting-started.html#setting-up-your-app	https://codesandbox.io/p/sandbox/admiring-wood-fqqj8h

While @mui/material has a built-in mechanism to handle the absence of a ThemeProvider, most tested libraries strictly require it to be set up for proper rendering and styling. The behavior varies across different libraries, with some failing to render components and others rendering unstyled components without a theme.

[jzempel]

@ze-flo I think this is pretty compelling evidence that you're on the right path with this PR. Can we get the migration.md updated with the breaking change to theming and either add a corresponding ADR or have a plan to add it immediately after?

[ze-flo]

@ze-flo I think this is pretty compelling evidence that you're on the right path with this PR. Can we get the migration.md updated with the breaking change to theming and either add a corresponding ADR or have a plan to add it immediately after?

I updated the migration docs and created a ticket to add the corresponding ADR in a follow-up PR.