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/IntroStep/IntroStep.tsx

@@ -65,7 +65,7 @@ function getSubheading(values: IntroStepValues): string {
  *
  * Entry point with urgency-sensitive segmentation. User selects who
  * the funeral is for, and (if arranging for someone else) whether
- * that person has died.
+ * that person has passed away.
  *
  * Uses the Centered Form layout variant. Progressive disclosure:
  * selecting "Someone else" reveals the hasPassedAway question.
@@ -117,9 +117,10 @@ export const IntroStep: React.FC<IntroStepProps> = ({
       <Box
         component="form"
         noValidate
+        aria-busy={loading}
         onSubmit={(e: React.FormEvent) => {
           e.preventDefault();
-          onContinue();
+          if (!loading) onContinue();
         }}
       >
         {/* forWhom field */}
@@ -151,7 +152,7 @@ export const IntroStep: React.FC<IntroStepProps> = ({
         <Collapse in={showHasPassedAway}>
           <Box sx={{ mb: 3 }}>
             <ToggleButtonGroup
-              label="Has the person died?"
+              label="Has this person passed away?"
               options={[
                 {
                   value: 'yes',