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/CoffinsStep/CoffinsStep.tsx

@@ -200,7 +200,7 @@ export const CoffinsStep: React.FC<CoffinsStepProps> = ({
             mb: 3,
           }}
         >
-          <FilterPanel activeCount={activeFilterCount} onClear={handleFilterClear} minWidth={300}>
+          <FilterPanel activeCount={activeFilterCount} onClear={handleFilterClear}>
             <TextField
               select
               label="Category"