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.

app/controllers/goals_controller.rb

@@ -71,20 +71,39 @@ class GoalsController < ApplicationController
   end
 
   def edit
+    @linkable_accounts = linkable_accounts_for_new
+    @currently_linked_account_ids = @goal.goal_accounts.pluck(:account_id).map(&:to_s)
   end
 
   def update
-    if @goal.update(goal_update_params)
-      flash[:notice] = t(".success")
-      respond_to do |format|
-        format.html { redirect_to goal_path(@goal) }
-        format.turbo_stream do
-          render turbo_stream: turbo_stream.action(:redirect, goal_path(@goal))
-        end
-      end
-    else
+    account_ids = params.dig(:goal, :account_ids)
+    accounts_supplied = !account_ids.nil?
+    accounts = accounts_supplied ? lookup_accounts(account_ids) : []
+
+    if accounts_supplied && accounts.empty?
+      @goal.errors.add(:base, :at_least_one_linked_account_required)
+      @linkable_accounts = linkable_accounts_for_new
+      @currently_linked_account_ids = @goal.goal_accounts.pluck(:account_id).map(&:to_s)
       render :edit, status: :unprocessable_entity
+      return
     end
+
+    Goal.transaction do
+      @goal.update!(goal_update_params)
+      sync_linked_accounts!(@goal, accounts) if accounts_supplied
+    end
+
+    flash[:notice] = t(".success")
+    respond_to do |format|
+      format.html { redirect_to goal_path(@goal) }
+      format.turbo_stream do
+        render turbo_stream: turbo_stream.action(:redirect, goal_path(@goal))
+      end
+    end
+  rescue ActiveRecord::RecordInvalid
+    @linkable_accounts = linkable_accounts_for_new
+    @currently_linked_account_ids = @goal.goal_accounts.pluck(:account_id).map(&:to_s)
+    render :edit, status: :unprocessable_entity
   end
 
   def destroy
@@ -148,6 +167,18 @@ class GoalsController < ApplicationController
       Current.family.accounts.where(accountable_type: "Depository").visible.alphabetically.to_a
     end
 
+    def sync_linked_accounts!(goal, accounts)
+      desired = accounts.map(&:id).to_set
+      current = goal.goal_accounts.pluck(:account_id).to_set
+
+      (current - desired).each do |id|
+        goal.goal_accounts.where(account_id: id).destroy_all
+      end
+      (desired - current).each do |id|
+        goal.goal_accounts.create!(account_id: id)
+      end
+    end
+
     def create_initial_contribution_if_provided!(goal, accounts)
       amount = params.dig(:goal, :initial_contribution_amount)
       account_id = params.dig(:goal, :initial_contribution_account_id)