feat: implement icon directive with relative path fallback

Refactor icon rendering to use a custom `marked-directive` instead of
a global icon prefix. This allows icons to use a CSS-based fallback
chain (searching up to 10 directory levels for an `assets/` folder),
making icon paths more robust and removing the need to pass path
prefixes through the component tree.

src/components/md-deck/CardLayer.tsx

@@ -4,7 +4,6 @@ import { getLayerStyle } from "./hooks/dimensions";
 import type { CardData, CardSide, LayerConfig } from "./types";
 import { DeckStore } from "./hooks/deckStore";
 import { processVariables } from "../utils/csv-loader";
-import { resolvePath } from "../utils/path";
 import type { LayerInteractionHandlers } from "./hooks/useLayerInteraction";
 
 export interface CardLayerProps {
@@ -26,13 +25,8 @@ export function CardLayer(props: CardLayerProps) {
   const draggingState = () => props.store.state.draggingState;
 
   function renderLayerContent(content: string) {
-    const iconPath = resolvePath(
-      props.store.state.cards.sourcePath,
-      props.cardData.iconPath ?? "./assets",
-    );
     return parseMarkdown(
       processVariables(content, props.cardData, props.store.state.cards),
-      iconPath,
     ) as string;
   }