fix: forward app shortcuts pressed inside the book

Key events do not cross document boundaries, so shortcuts bound on the window
never fired while the book had focus and the browser default won instead —
ctrl+B opened bookmarks rather than the chapter sidebar. Replay them on the
window, cancelling the original only if a handler claimed it so ctrl+C still
copies.

frontend/src/lib/components/reader/foliate-view.svelte

@@ -104,7 +104,46 @@
 		doc.addEventListener('keydown', onKeydown);
 	}
 
+	/**
+	 * Replays a shortcut pressed inside the book on the host window.
+	 *
+	 * Key events do not cross document boundaries, so a shortcut pressed while
+	 * the book has focus never reaches the app's handlers — which are bound on
+	 * the window, as the sidebar's ctrl+B is — and the browser's own default runs
+	 * instead. That is why ctrl+B opened bookmarks in the book but toggled the
+	 * chapter sidebar everywhere else.
+	 *
+	 * The original is only cancelled if an app handler actually claimed the
+	 * replay, so combinations the app does not use — ctrl+C above all — keep
+	 * their normal browser behaviour.
+	 */
+	function forwardShortcut(event: KeyboardEvent) {
+		const source = (event.target as Node | null)?.ownerDocument;
+		// isTrusted rules out the replay itself, which would otherwise recurse.
+		if (!event.isTrusted || !source || source === document) return;
+
+		const claimed = !window.dispatchEvent(
+			new KeyboardEvent('keydown', {
+				key: event.key,
+				code: event.code,
+				ctrlKey: event.ctrlKey,
+				metaKey: event.metaKey,
+				shiftKey: event.shiftKey,
+				altKey: event.altKey,
+				bubbles: true,
+				cancelable: true
+			})
+		);
+
+		if (claimed) event.preventDefault();
+	}
+
 	function onKeydown(event: KeyboardEvent) {
+		if (event.ctrlKey || event.metaKey || event.altKey) {
+			forwardShortcut(event);
+			return;
+		}
+
 		switch (event.key) {
 			case 'ArrowLeft':
 			case 'PageUp':