Fix nil-key collision in budget category hash lookups (#1136)

Both Uncategorized and Other Investments are synthetic categories with
id=nil. When expense_totals_by_category indexes by category.id, Other
Investments overwrites Uncategorized at the nil key, causing uncategorized
actual spending to always return 0.

Use category.name as fallback key (id || name) to differentiate the two
synthetic categories in all hash builders and lookup sites.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

test/models/budget_test.rb

@@ -304,4 +304,30 @@ class BudgetTest < ActiveSupport::TestCase
 
     assert_not_nil budget.previous_budget_param
   end
+
+  test "uncategorized budget category actual spending reflects uncategorized transactions" do
+    family = families(:dylan_family)
+    budget = Budget.find_or_bootstrap(family, start_date: Date.current.beginning_of_month)
+    account = accounts(:depository)
+
+    # Create an uncategorized expense
+    Entry.create!(
+      account: account,
+      entryable: Transaction.create!(category: nil),
+      date: Date.current,
+      name: "Uncategorized lunch",
+      amount: 75,
+      currency: "USD"
+    )
+
+    budget = Budget.find(budget.id)
+    budget.sync_budget_categories
+
+    uncategorized_bc = budget.uncategorized_budget_category
+    spending = budget.budget_category_actual_spending(uncategorized_bc)
+
+    # Must be > 0 — the nil-key collision between Uncategorized and
+    # Other Investments synthetic categories previously caused this to return 0
+    assert spending >= 75, "Uncategorized actual spending should include the $75 transaction, got #{spending}"
+  end
 end