fix(config): cap custom font size at 20rem (@Cyronite)

[Cyronite]

Description

Custom font size accepted any positive number, and allowed extreme cases (eg. 100000) making the website unusable. This change adds a max of 20rem to the shared FontSizeSchema. The settings and command palette reject any values larger than this through the already built validation UI. The previously saved invalid sizes fall back to the default when settings are loaded.

Added regression tests for valid sizes, rejected extreme values, and recovery of saved settings. All 128 relevant tests passed, along with formatting and type-aware lint.

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 #8454