docs: add 15-improvements design doc and implementation plan

Also tracks Cargo.lock and BoardCardOverlay component from prior sessions.

src/components/boards/BoardCardOverlay.tsx

@@ -0,0 +1,34 @@
+import { formatDistanceToNow } from "date-fns";
+import type { BoardMeta } from "@/types/board";
+
+interface BoardCardOverlayProps {
+  board: BoardMeta;
+}
+
+export function BoardCardOverlay({ board }: BoardCardOverlayProps) {
+  const relativeTime = formatDistanceToNow(new Date(board.updatedAt), {
+    addSuffix: true,
+  });
+
+  return (
+    <div className="flex w-full flex-col rounded-lg bg-pylon-surface shadow-xl ring-2 ring-pylon-accent/40 opacity-90 cursor-grabbing">
+      {/* Color accent stripe */}
+      <div
+        className="h-1 w-full rounded-t-lg"
+        style={{ backgroundColor: board.color }}
+      />
+      <div className="flex flex-col gap-2 p-4">
+        <h3 className="font-heading text-lg text-pylon-text">
+          {board.title}
+        </h3>
+        <p className="font-mono text-xs text-pylon-text-secondary">
+          {board.cardCount} card{board.cardCount !== 1 ? "s" : ""} &middot;{" "}
+          {board.columnCount} column{board.columnCount !== 1 ? "s" : ""}
+        </p>
+        <p className="font-mono text-xs text-pylon-text-secondary">
+          {relativeTime}
+        </p>
+      </div>
+    </div>
+  );
+}