fix: knowledge Base Table — Row Icon Appears Clipped/Cut for Some Entries (#12009)

* removed book and added file. makes more sense

* feat: add accent-blue color to design system and update knowledge base file icon

- Add accent-blue color variables to light and dark themes in CSS
- Register accent-blue in Tailwind config with DEFAULT and foreground variants
- Update knowledge base file icon fallback color from hardcoded text-blue-500 to text-accent-blue-foreground