Keep provider settings above the model selector (#2476)

* fix(app): keep provider settings above model selector

Desktop web comboboxes used the browser top layer, so ordinary modal portals could not cover them. Keep web overlays in one relative layer stack while preserving native bottom-sheet stacking.

* fix(app): route overlay keyboard ownership

* fix(app): preserve nested overlay ownership

* fix(app): route command center overlay input

* fix(app): preserve global overlay ancestry

* fix(app): register global dialog hosts

packages/app/e2e/new-workspace-codex-mode-preferences.spec.ts

@@ -100,11 +100,7 @@ async function selectMode(page: Page, label: string): Promise<void> {
   await expect(searchInput).toBeVisible({ timeout: 10_000 });
   await searchInput.fill(label);
 
-  const option = page
-    .getByRole("dialog")
-    .last()
-    .getByText(new RegExp(`^${escapeRegex(label)}$`, "i"))
-    .first();
+  const option = popup.getByText(new RegExp(`^${escapeRegex(label)}$`, "i")).first();
   await expect(option).toBeVisible({ timeout: 10_000 });
   await option.click({ force: true });
   await expect(searchInput).not.toBeVisible({ timeout: 5_000 });