fix(savings_goals/new): trap Enter on step 1; add funding-accounts hint

- Pressing Enter inside a step-1 input (Name, Target amount, Target
  date) used to fire the form-implicit-submission against the sr-only
  submit button, jumping straight to POST /savings_goals and skipping
  step 2 entirely (no initial contribution, no review).
- New blockEnter action on the form re-routes Enter to next() when
  currentStep === 1, mirroring the Continue button. Notes textarea is
  exempt so newlines work.
- Add an inline hint under the funding-accounts label so users know up
  front what the field controls; previously the only feedback was a
  tiny "must pick one" error after Continue.

app/javascript/controllers/savings_goal_stepper_controller.js

@@ -49,6 +49,15 @@ export default class extends Controller {
     this.refreshSubmitState();
   }
 
+  blockEnter(event) {
+    if (this.currentStep !== 1) return;
+    // Allow Enter in the notes textarea so newlines work.
+    if (event.target.tagName === "TEXTAREA") return;
+    event.preventDefault();
+    // Mirror Continue: validate + advance instead of swallowing silently.
+    this.next();
+  }
+
   footerLeft(event) {
     event.preventDefault();
     this.back();