chore: add knip, tighten ESLint, and clean up dead code

[hazre]

Description

It adds Knip, migrates and tightens the lint setup, removes dead code, and fixes the resulting lint issues across the app while keeping runtime behavior intact.

• migrate ESLint to the airbnb-extended flat config
• convert shared utilities like colorMXID to TypeScript and standardize shared utility imports/aliases

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings