feat(goals/edit): funding-accounts editor in the edit modal

Previously a user who linked the wrong account at creation had to
delete + recreate the goal. Now the edit modal carries the same
funding-accounts checkbox group as Step 1 of the stepper, pre-checked
with the goal's current links.

- GoalsController#edit loads @linkable_accounts + @currently_linked_account_ids.
- #update accepts account_ids; when supplied, runs the create / update
  inside a Goal.transaction and syncs linked accounts via
  sync_linked_accounts! (set-diff: destroy_all unselected goal_accounts,
  create the new ones). Validates at least one account before touching
  goal_accounts so the user gets a clean re-render.
- Removing an account preserves the goal's existing contributions —
  GoalContribution#account_must_be_linked_to_goal only fires on save,
  so historical rows stay valid.
- _form_edit partial accepts new locals; edit.html.erb threads them
  through.
- 3 new controller tests: identity-only patch leaves links intact;
  account_ids patch replaces the link set; empty account_ids
  re-renders with error.

test/controllers/goals_controller_test.rb

@@ -104,6 +104,25 @@ class GoalsControllerTest < ActionDispatch::IntegrationTest
     assert_equal "Renamed", @goal.reload.name
   end
 
+  test "update without account_ids leaves linked accounts intact" do
+    before = @goal.goal_accounts.pluck(:account_id).sort
+    patch goal_url(@goal), params: { goal: { name: "Still here" } }
+    assert_redirected_to goal_path(@goal)
+    assert_equal before, @goal.reload.goal_accounts.pluck(:account_id).sort
+  end
+
+  test "update with account_ids syncs linked accounts (add + remove)" do
+    patch goal_url(@goal), params: { goal: { account_ids: [ @connected.id ] } }
+    assert_redirected_to goal_path(@goal)
+    assert_equal [ @connected.id ], @goal.reload.goal_accounts.pluck(:account_id)
+  end
+
+  test "update with empty account_ids re-renders with error" do
+    patch goal_url(@goal), params: { goal: { account_ids: [ "" ] } }
+    assert_response :unprocessable_entity
+    assert_not_empty @goal.reload.goal_accounts
+  end
+
   test "pause/resume/complete/archive/unarchive flow" do
     fresh = goals(:emergency_fund)
     patch pause_goal_url(fresh)