feat(retirement): PR4d what-if slider rail

Each lever (retire age / target spend / save per mo / real return) now
pairs a numeric input with a range slider; retirement_what_if_controller
mirrors the value across the pair (data-lever) and debounces the live
forecast preview. Birth year stays a plain numeric input.

(Skinned delete confirmations already render via Sure's global
Turbo.config.forms.confirm → DS::Dialog override, so the PR2 turbo_confirm
buttons are already styled — no change needed.)

app/javascript/controllers/retirement_what_if_controller.js

@@ -7,6 +7,18 @@ export default class extends Controller {
   static targets = ["form"]
   static values = { url: String, debounce: { type: Number, default: 300 } }
 
+  // Mirror a lever's value across its paired number + range inputs (matched
+  // by data-lever), then debounce a preview.
+  sync(event) {
+    const lever = event.target.dataset.lever
+    if (lever) {
+      this.element.querySelectorAll(`[data-lever="${lever}"]`).forEach((el) => {
+        if (el !== event.target) el.value = event.target.value
+      })
+    }
+    this.preview()
+  }
+
   preview() {
     clearTimeout(this.timer)
     this.timer = setTimeout(() => this.fetchPreview(), this.debounceValue)