feat(theme): add Sakura theme (@meowster404)

[meowster404]

Description

This PR adds a new theme called Sakura, inspired by cherry blossoms and soft pastel colors.

The theme uses a light pink background with darker burgundy text to maintain readability. The main color was intentionally chosen to provide better key visibility and contrast against the background, improving the typing experience while keeping the overall soft aesthetic. Warm gold is used for the caret to provide a clear focus point, and the error colors are designed to remain noticeable without clashing with the palette.

Checks

• Adding quotes?
  • Make sure to follow the quotes documentation
  • Make sure to include translations for the quotes in the description (or another comment) so we can verify their content.
• Adding a language?
  • Make sure to follow the languages documentation
  • Add language to packages/schemas/src/languages.ts
  • Add language to exactly one group in frontend/src/ts/constants/languages.ts
  • Add language json file to frontend/static/languages
• Adding a theme?
  • Make sure to follow the themes documentation
  • Add theme to packages/schemas/src/themes.ts
  • Add theme to frontend/src/ts/constants/themes.ts
  • (optional) Add theme css file to frontend/static/themes
  • Add some screenshots of the theme, especially with different test settings (colorful, flip colors) to your pull request
• Adding a layout?
  • Make sure to follow the layouts documentation
  • Add layout to packages/schemas/src/layouts.ts
  • Add layout json file to frontend/static/layouts
• Adding a font?
  • Make sure to follow the fonts documentation
  • Add font file to frontend/static/webfonts
  • Add font to packages/schemas/src/fonts.ts
  • Add font to frontend/src/ts/constants/fonts.ts
• Check if any open issues are related to this PR; if so, be sure to tag them below.
• Make sure the PR title follows the Conventional Commits standard. (https://www.conventionalcommits.org for more info)
• Make sure to include your GitHub username prefixed with @ inside parentheses at the end of the PR title.

Closes #