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/routes/threads/$threadId.tsx

@@ -38,7 +38,7 @@ function ThreadDetailPage() {
 				<Link
 					to="/"
 					search={{ tab: "planning" }}
-					className="text-sm text-blue-600 hover:text-blue-700"
+					className="text-sm text-gray-600 hover:text-gray-700"
 				>
 					Back to planning
 				</Link>
@@ -67,7 +67,7 @@ function ThreadDetailPage() {
 					<span
 						className={`inline-flex items-center px-2 py-0.5 rounded-full text-xs font-medium ${
 							isActive
-								? "bg-blue-50 text-blue-700"
+								? "bg-gray-100 text-gray-600"
 								: "bg-gray-100 text-gray-500"
 						}`}
 					>
@@ -92,7 +92,7 @@ function ThreadDetailPage() {
 					<button
 						type="button"
 						onClick={openCandidateAddPanel}
-						className="inline-flex items-center gap-2 px-4 py-2 bg-blue-600 hover:bg-blue-700 text-white text-sm font-medium rounded-lg transition-colors"
+						className="inline-flex items-center gap-2 px-4 py-2 bg-gray-700 hover:bg-gray-800 text-white text-sm font-medium rounded-lg transition-colors"
 					>
 						<svg
 							className="w-4 h-4"