style: apply prettier formatting and fix remaining Eva light elements

[joaquinlarroca]

Description

Added Prettier as the project's formatter (.prettierrc with tabWidth: 4) and applied it across all CSS, JS, HTML and markdown files. Alongside the formatting pass, this fixes remaining light elements in EVA dark mode: activity completion buttons (btn-success), white-background tables and rows, and muted text/borders — consistent with existing --inkFing-* variables. Also runs image theming on the /index.php front page path.

Type of change

• Bug fix
• New feature
• Improvement of existing code
• Documentation
• Other: **____**

How did you test the changes?

• I ran the app locally and tested the new feature
• I ran the tests and they pass

Checklist

Before submitting your PR, verify:

• My code follows project conventions
• I added comments where code is not self-explanatory
• I updated documentation if necessary
• There are no errors or warnings in the console
• My commit message follows the conventional format

---

Thanks for contributing! Your PR will be reviewed by a maintainer.