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.
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