Fiks fullskjerm: portal til document.body for å unngå Canvas transform

Canvas sin CSS transform bryter position:fixed (kjent CSS-gotcha).
Fullskjerm flytter nå DOM-elementet til body, og tilbake ved exit.
z-index 9999 og bakgrunnsfarge #0a0a0b.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/blockshell/BlockShell.svelte

@@ -126,8 +126,29 @@
 	});
 
 	// --- Fullscreen toggle ---
+	// Fullscreen panels must escape Canvas's transform context,
+	// which breaks position:fixed. We move the DOM element to
+	// document.body when entering fullscreen, and back when exiting.
+	let originalParent: HTMLElement | null = null;
+	let originalNextSibling: Node | null = null;
+
 	function toggleFullscreen() {
 		isFullscreen = !isFullscreen;
+		if (containerEl) {
+			if (isFullscreen) {
+				originalParent = containerEl.parentElement;
+				originalNextSibling = containerEl.nextSibling;
+				document.body.appendChild(containerEl);
+			} else if (originalParent) {
+				if (originalNextSibling) {
+					originalParent.insertBefore(containerEl, originalNextSibling);
+				} else {
+					originalParent.appendChild(containerEl);
+				}
+				originalParent = null;
+				originalNextSibling = null;
+			}
+		}
 		onFullscreenChange?.(isFullscreen);
 	}
 
@@ -438,11 +459,12 @@
 	.blockshell-fullscreen {
 		position: fixed;
 		inset: 0;
-		z-index: 50;
+		z-index: 9999;
 		width: 100% !important;
 		height: 100% !important;
 		border-radius: 0;
 		border: none;
+		background: #0a0a0b;
 	}
 
 	.blockshell-minimized {