Quality pass: fix P0 audit findings across FilterPanel, ArrangementDialog, steps

FilterPanel (4 P0 + 2 P1 fixes):
- Add forwardRef (project convention)
- Use React.useId() for unique popover/heading IDs (was static)
- Change aria-describedby to aria-controls (correct ARIA pattern)
- Add role="dialog" + aria-labelledby on Popover paper
- Popover header now uses label prop (was hardcoded "Filters")
- Clear all font size uses theme.typography.caption (was hardcoded)
- Badge uses aria-hidden + visually-hidden text (cleaner SR output)
- Add maxHeight + overflow scroll to body, aria-label on Done button

ArrangementDialog (3 P0 + 1 P1 fixes):
- Add forwardRef
- Focus management: titleRef focused on step change via useEffect
- Add aria-live region announcing step transitions to screen readers
- Fix borderRadius from 3 to 2 (theme convention)

Sticky header padding (visual fix):
- ProvidersStep + VenueStep: mx/px now responsive { xs: -2/2, md: -3/3 }
  matching the panel's px: { xs: 2, md: 3 } — fixes mobile misalignment

CoffinDetailsStep:
- Wrap CTA area in form element with onSubmit + aria-busy

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

src/components/pages/VenueStep/VenueStep.tsx

@@ -214,8 +214,8 @@ export const VenueStep: React.FC<VenueStepProps> = ({
             zIndex: 1,
             bgcolor: 'background.default',
             pb: 1,
-            mx: -3,
-            px: 3,
+            mx: { xs: -2, md: -3 },
+            px: { xs: 2, md: 3 },
           }}
         >
           <Typography variant="display3" component="h1" sx={{ mb: 0.5 }} tabIndex={-1}>