Feedback iteration: DialogShell, page consistency, popup standardisation

- Add DialogShell atom — shared dialog container (header, scrollable body, footer)
- Refactor FilterPanel to use DialogShell (Popover → centered Dialog)
- Refactor ArrangementDialog to use DialogShell
- Remove PreviewStep + AuthGateStep pages (consolidated into ArrangementDialog, D-E)
- IntroStep: static subheading, top-left aligned toggle button content
- ProvidersStep: h4 heading "Find a funeral director", location search with pin icon,
  filter moved below search right-aligned, map fill fix, hover scrollbar
- VenueStep: same consistency fixes (h4 heading, filter layout, location icon, map fix)
- PackagesStep: grouped packages ("Matching your preferences" / "Other packages from
  [Provider]"), removed budget filter + Most Popular badge, clickable provider card,
  onArrange replaces onContinue, h4 heading
- WizardLayout: list-map left panel gets thin scrollbar visible on hover

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/pages/IntroStep/IntroStep.tsx

@@ -46,17 +46,10 @@ export interface IntroStepProps {
   sx?: SxProps<Theme>;
 }
 
-// ─── Copy helpers ────────────────────────────────────────────────────────────
+// ─── Constants ──────────────────────────────────────────────────────────────
 
-function getSubheading(values: IntroStepValues): string {
-  if (values.forWhom === 'someone' && values.hasPassedAway === 'yes') {
-    return "We'll guide you through each step. You can save your progress and come back anytime.";
-  }
-  if (values.forWhom === 'myself' || values.hasPassedAway === 'no') {
-    return "Explore your options and plan at your own pace. Nothing is locked in until you're ready.";
-  }
-  return "We'll guide you through arranging a funeral, step by step. You can save your progress and come back anytime.";
-}
+const SUBHEADING =
+  "We'll guide you through arranging a funeral, step by step. You can save your progress and come back anytime.";
 
 // ─── Component ───────────────────────────────────────────────────────────────
 
@@ -110,8 +103,8 @@ export const IntroStep: React.FC<IntroStepProps> = ({
         Let&apos;s get started
       </Typography>
 
-      <Typography variant="body1" color="text.secondary" sx={{ mb: 5 }} aria-live="polite">
-        {getSubheading(values)}
+      <Typography variant="body1" color="text.secondary" sx={{ mb: 5 }}>
+        {SUBHEADING}
       </Typography>
 
       <Box