Improve desktop command autocomplete to match combobox behavior (#53)

* feat(app): align command autocomplete with desktop combobox

* refactor(app): extract shared autocomplete UI and command hook

* refactor(app): rename autocomplete hooks to generic naming

packages/app/src/components/ui/autocomplete-utils.test.ts

@@ -0,0 +1,62 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  getAutocompleteFallbackIndex,
+  getAutocompleteScrollOffset,
+  orderAutocompleteOptions,
+} from "./autocomplete-utils";
+
+const OPTIONS = ["alpha", "beta", "gamma"];
+
+describe("orderAutocompleteOptions", () => {
+  it("keeps first logical option closest to the input by default", () => {
+    expect(orderAutocompleteOptions(OPTIONS)).toEqual([
+      "gamma",
+      "beta",
+      "alpha",
+    ]);
+  });
+
+  it("keeps normal top-down order when below-input is selected", () => {
+    expect(orderAutocompleteOptions(OPTIONS, "below-input")).toEqual([
+      "alpha",
+      "beta",
+      "gamma",
+    ]);
+  });
+});
+
+describe("getAutocompleteFallbackIndex", () => {
+  it("picks the option nearest the input by default", () => {
+    expect(getAutocompleteFallbackIndex(3)).toBe(2);
+    expect(getAutocompleteFallbackIndex(0)).toBe(-1);
+  });
+
+  it("picks top item when below-input ordering is used", () => {
+    expect(getAutocompleteFallbackIndex(3, "below-input")).toBe(0);
+  });
+});
+
+describe("getAutocompleteScrollOffset", () => {
+  it("scrolls up when the active item is above the viewport", () => {
+    expect(
+      getAutocompleteScrollOffset({
+        currentOffset: 120,
+        viewportHeight: 80,
+        itemTop: 90,
+        itemHeight: 20,
+      })
+    ).toBe(90);
+  });
+
+  it("scrolls down when the active item is below the viewport", () => {
+    expect(
+      getAutocompleteScrollOffset({
+        currentOffset: 0,
+        viewportHeight: 100,
+        itemTop: 150,
+        itemHeight: 24,
+      })
+    ).toBe(74);
+  });
+});