Fix desktop layout and window controls at half-screen width (#1983)

* fix(app): preserve desktop layout in half-screen windows

Treat native window controls as physical corner obstructions instead of deriving clearance from compact layout state. Keep 751px windows in desktop mode while preserving usable content when persisted sidebars are wide.

* fix(app): retain sidebar width across window resizes

* fix(app): yield app sidebar to narrow settings

* test(app): wait for mobile sidebar motion to settle

* fix(app): yield navigation to narrow workspace explorer

* fix(app): mark sidebar sizing helpers as worklets

* fix(app): hide app navigation when space is constrained

packages/app/src/components/split-container-focus.ts

@@ -0,0 +1,21 @@
+import type { SplitNode, SplitPane } from "@/stores/workspace-layout-store";
+
+export function resolveSplitContainerRoot(input: {
+  root: SplitNode;
+  focusedPaneId: string | null;
+  focusModeEnabled: boolean | undefined;
+}): { root: SplitNode; usesFallbackStrip: boolean } {
+  if (!input.focusModeEnabled) return { root: input.root, usesFallbackStrip: false };
+  const focusedPane = input.focusedPaneId ? findPane(input.root, input.focusedPaneId) : null;
+  if (!focusedPane) return { root: input.root, usesFallbackStrip: true };
+  return { root: { kind: "pane", pane: focusedPane }, usesFallbackStrip: false };
+}
+
+function findPane(node: SplitNode, paneId: string): SplitPane | null {
+  if (node.kind === "pane") return node.pane.id === paneId ? node.pane : null;
+  for (const child of node.group.children) {
+    const pane = findPane(child, paneId);
+    if (pane) return pane;
+  }
+  return null;
+}