chore: replace hardcoded hex colors with named Tailwind classes in folder-icon

Problem

apps/web/ui/folders/folder-icon.tsx uses four hardcoded hex color values in defaultIconClassName that map exactly to standard Tailwind color names. Three of the five folder types already use proper named classes (text-blue-800); the other three use bare hex strings:

Hex	Tailwind equivalent
text-[#3730A3]	text-indigo-800
text-[#9A3412]	text-orange-800
text-[#1F2937]	text-gray-800
text-[#166534]	text-green-800

This is the same pattern addressed in #3312.

Fix

Replace the four hex strings with their named equivalents so the file is consistent (all five folder types now use the color-800 naming pattern) and Tailwind's tooling (IntelliSense, tree-shaking, theming) can work correctly.

I have a one-line-per-color fix ready to PR.