fix(ux): popover collision padding, PWA manifest, webhook toasts, portal toast, dashboard error boundary, GDPR poll backoff, empty-state CTA

Grab-bag of UX gaps from audit-pass-#2 + #3. Each one is a small,
focused fix; bundled because they touch different surfaces.

- Popover: collisionPadding={16} + responsive
  w-[min(calc(100vw-2rem),18rem)] so popovers stop clipping past the
  viewport on iPhone 12 portrait.
- public/manifest.json (was missing) + manifest reference in
  layout.tsx — PWA installability now works; icons (192/512/512-
  maskable) were already present.
- Admin webhooks page: 4 silent `// ignore` catches in load/delete/
  toggle/regenerate replaced with toast.error / toast.success. Users
  no longer see a stale list with no feedback when an op fails.
- Portal document-download button: blocking alert() → toast.error().
- src/app/(dashboard)/error.tsx: branded error boundary with retry +
  back-to-dashboard, replacing Next.js's default uncaught-error UI.
- GDPR export modal: refetchInterval was a flat 5s while the modal was
  open. Switched to a function that only polls (every 15s) when a job
  is actually pending/building; settled exports stop polling entirely.
- client-yachts-tab empty state gains a CTA wired to the existing
  Add-yacht dialog, instead of just saying "No yachts".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/(portal)/portal/documents/document-download-button.tsx

@@ -2,6 +2,8 @@
 
 import { useState } from 'react';
 import { Download, Loader2 } from 'lucide-react';
+import { toast } from 'sonner';
+
 import { Button } from '@/components/ui/button';
 
 interface DocumentDownloadButtonProps {
@@ -16,25 +18,20 @@ export function DocumentDownloadButton({ documentId }: DocumentDownloadButtonPro
     try {
       const res = await fetch(`/api/portal/documents/${documentId}/download`);
       if (!res.ok) {
-        alert('Unable to download document. Please try again.');
+        toast.error('Unable to download document. Please try again.');
         return;
       }
-      const data = await res.json() as { url: string };
+      const data = (await res.json()) as { url: string };
       window.open(data.url, '_blank', 'noopener,noreferrer');
     } catch {
-      alert('Unable to download document. Please check your connection.');
+      toast.error('Unable to download document. Please check your connection.');
     } finally {
       setLoading(false);
     }
   }
 
   return (
-    <Button
-      variant="outline"
-      size="sm"
-      onClick={handleDownload}
-      disabled={loading}
-    >
+    <Button variant="outline" size="sm" onClick={handleDownload} disabled={loading}>
       {loading ? (
         <Loader2 className="h-3.5 w-3.5 animate-spin" />
       ) : (