style: replace blue accent with gray and mute card badge colors

Switch all interactive UI elements (buttons, focus rings, active tabs,
FAB, links, spinners) from blue to gray to match icon colors for a
more cohesive look. Mute card badge text colors to pastels (blue-400,
green-500, purple-500) to keep the focus on card content.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/client/components/CategoryHeader.tsx

@@ -64,7 +64,7 @@ export function CategoryHeader({
 				<button
 					type="button"
 					onClick={handleSave}
-					className="text-sm text-blue-600 hover:text-blue-800 font-medium"
+					className="text-sm text-gray-600 hover:text-gray-800 font-medium"
 				>
 					Save
 				</button>