polish(categories): honest doc + extract/test picker-options glue (#246 review)

Address the two non-blocking review notes on PR #252:
- The re-injected non-leaf parent (e.g. #1710) is technically clickable, not
  'never offered'; correct the MappingRow comment to state selecting it is a
  no-op (only leaves resolve).
- Extract the per-row option-building glue into a pure, unit-tested helper
  comboboxCategoriesForTarget (4 cases: null/leaf pass-through by reference,
  non-leaf append, unresolvable stale id).

src/components/categories-migration/MappingRow.tsx

@@ -1,6 +1,7 @@
 import { useMemo } from "react";
 import { useTranslation } from "react-i18next";
 import CategoryCombobox from "../shared/CategoryCombobox";
+import { comboboxCategoriesForTarget } from "./migrationTargets";
 import type { Category } from "../../shared/types";
 import type {
   MappingRow as MappingRowType,
@@ -64,15 +65,13 @@ export default function MappingRow({
   // The picker lists leaves only. If this row's current target is a non-leaf
   // parent (a low-confidence default like "Divertissement" #1710), it is absent
   // from that list and would blank the combobox input, so re-inject it for this
-  // row only — shown as the current value, never offered as a new pick.
-  const comboboxCategories = useMemo(() => {
-    const id = row.v1TargetId;
-    if (id == null || targetCategories.some((c) => c.id === id)) {
-      return targetCategories;
-    }
-    const current = resolveTarget(id);
-    return current ? [...targetCategories, current] : targetCategories;
-  }, [targetCategories, row.v1TargetId, resolveTarget]);
+  // row so it stays visible. It remains clickable, but selecting it is a no-op —
+  // handleTargetChange only resolves ids present in `targetCategories` (leaves) —
+  // so any pick that takes effect is a leaf. See comboboxCategoriesForTarget.
+  const comboboxCategories = useMemo(
+    () => comboboxCategoriesForTarget(targetCategories, row.v1TargetId ?? null, resolveTarget),
+    [targetCategories, row.v1TargetId, resolveTarget],
+  );
 
   const badgeLabel = t(
     `categoriesSeed.migration.simulate.confidence.${row.confidence}`,

src/components/categories-migration/migrationTargets.test.ts

@@ -2,7 +2,9 @@ import { describe, it, expect } from "vitest";
 import {
   taxonomyToComboboxCategories,
   findTaxonomyCategory,
+  comboboxCategoriesForTarget,
 } from "./migrationTargets";
+import type { Category } from "../../shared/types";
 import {
   getTaxonomyV1,
   type TaxonomyNode,
@@ -127,3 +129,49 @@ describe("findTaxonomyCategory", () => {
     expect(c).toMatchObject({ id: 1710, is_inputable: false });
   });
 });
+
+describe("comboboxCategoriesForTarget", () => {
+  const leaf = (id: number): Category => ({
+    id,
+    name: `Leaf ${id}`,
+    parent_id: undefined,
+    type: "expense",
+    is_active: true,
+    is_inputable: true,
+    sort_order: id,
+    created_at: "",
+  });
+  const leaves = [leaf(1111), leaf(1121), leaf(1131)];
+  const parent1710: Category = {
+    id: 1710,
+    name: "Divertissement",
+    parent_id: undefined,
+    type: "expense",
+    is_active: true,
+    is_inputable: false,
+    sort_order: Number.MAX_SAFE_INTEGER,
+    created_at: "",
+  };
+  const resolveTarget = (id: number) => (id === 1710 ? parent1710 : null);
+
+  it("returns the leaves list unchanged (same reference) when target is null", () => {
+    expect(comboboxCategoriesForTarget(leaves, null, resolveTarget)).toBe(leaves);
+  });
+
+  it("returns the leaves list unchanged (same reference) when target is already a leaf", () => {
+    expect(comboboxCategoriesForTarget(leaves, 1121, resolveTarget)).toBe(leaves);
+  });
+
+  it("appends the resolved non-leaf parent when the target is a parent absent from leaves", () => {
+    const out = comboboxCategoriesForTarget(leaves, 1710, resolveTarget);
+    expect(out).toHaveLength(leaves.length + 1);
+    expect(out[out.length - 1]).toMatchObject({ id: 1710, is_inputable: false });
+    // originals preserved, in order
+    expect(out.slice(0, 3).map((c) => c.id)).toEqual([1111, 1121, 1131]);
+  });
+
+  it("returns the leaves list unchanged when the non-leaf target cannot be resolved", () => {
+    // targetId absent from leaves AND resolver returns null (stale id) -> no injection
+    expect(comboboxCategoriesForTarget(leaves, 9999, resolveTarget)).toBe(leaves);
+  });
+});

src/components/categories-migration/migrationTargets.ts

@@ -79,3 +79,32 @@ export function findTaxonomyCategory(
   }
   return null;
 }
+
+/**
+ * Build the target-picker option list for one migration row.
+ *
+ * Returns the leaves-only `leafOptions` unchanged when the row's current target
+ * is null or already a leaf (the common case — same array reference, so the
+ * combobox never re-renders needlessly).
+ *
+ * When the target is a NON-leaf parent absent from the leaves list (a
+ * low-confidence default like "Divertissement" #1710), the resolved parent is
+ * appended so the combobox can DISPLAY it as the current value. It stays visible
+ * (and, since `CategoryCombobox` renders every option, clickable) but selecting
+ * it is a no-op: the row's resolve handler only accepts ids present in the
+ * leaves list, so re-picking the parent changes nothing and the user is steered
+ * to a leaf. New picks that take effect are therefore always leaves.
+ *
+ * Pure helper — no React, no i18n, no DB.
+ */
+export function comboboxCategoriesForTarget(
+  leafOptions: Category[],
+  targetId: number | null,
+  resolveTarget: (id: number) => Category | null,
+): Category[] {
+  if (targetId == null || leafOptions.some((c) => c.id === targetId)) {
+    return leafOptions;
+  }
+  const current = resolveTarget(targetId);
+  return current ? [...leafOptions, current] : leafOptions;
+}