Groom wizard steps 1-15: critique/harden/polish pass

- [P0] CrematoriumStep: Fix <option> → <MenuItem> in priority select
- [P1] All form steps: Add aria-busy={loading} + loading guard on submit
- [P1] Error messages: Replace color="error" (red) with copper
  (var(--fa-color-text-brand)) across ProvidersStep, PackagesStep,
  VenueStep, CrematoriumStep, CemeteryStep, CoffinsStep, PaymentStep
- [P2] IntroStep: "Has the person died?" → "Has this person passed away?"
- [P2] DateTimeStep: "About the person who died" → "who has passed"
- [P2] ProvidersStep: "Showing results from X" → "X providers found"
- [P2] Empty states: Add guidance text for ProvidersStep, PackagesStep,
  VenueStep, CoffinsStep empty results

Steps 4, 13, 15 passed with no issues.

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

src/components/pages/CoffinsStep/CoffinsStep.tsx

@@ -191,9 +191,10 @@ export const CoffinsStep: React.FC<CoffinsStepProps> = ({
     <Box
       component="form"
       noValidate
+      aria-busy={loading}
       onSubmit={(e: React.FormEvent) => {
         e.preventDefault();
-        onContinue();
+        if (!loading) onContinue();
       }}
     >
       {/* Page heading */}
@@ -293,11 +294,26 @@ export const CoffinsStep: React.FC<CoffinsStepProps> = ({
             </Box>
           </Card>
         ))}
+
+        {coffins.length === 0 && (
+          <Box sx={{ py: 6, textAlign: 'center', gridColumn: '1 / -1' }}>
+            <Typography variant="body1" color="text.secondary" sx={{ mb: 1 }}>
+              No coffins match your selected filters.
+            </Typography>
+            <Typography variant="body2" color="text.secondary">
+              Try adjusting the category or price range.
+            </Typography>
+          </Box>
+        )}
       </Box>
 
       {/* Validation error */}
       {errors?.selectedCoffinId && (
-        <Typography variant="body2" color="error" sx={{ mb: 2 }} role="alert">
+        <Typography
+          variant="body2"
+          sx={{ mb: 2, color: 'var(--fa-color-text-brand)' }}
+          role="alert"
+        >
           {errors.selectedCoffinId}
         </Typography>
       )}