fix: handle invalid date formats gracefully and add missing dialog descriptions

- Wrap date.fromisoformat() in try/except in case_update tool — prevents
  unhandled ValueError from surfacing as 500; FastAPI now catches it as 422
- Add DialogDescription (sr-only) to 5 dialogs missing aria-describedby:
  documents-panel preview + delete, drafts-panel delete + feedback, link-related-dialog

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web-ui/src/components/precedents/link-related-dialog.tsx

@@ -6,6 +6,7 @@ import { toast } from "sonner";
 import {
   Dialog,
   DialogContent,
+  DialogDescription,
   DialogHeader,
   DialogTitle,
 } from "@/components/ui/dialog";
@@ -68,6 +69,7 @@ function LinkDialog({ caseId, currentRelated, open, onOpenChange }: DialogProps)
       <DialogContent className="max-w-lg" dir="rtl">
         <DialogHeader>
           <DialogTitle className="text-navy">קשר החלטה קשורה</DialogTitle>
+          <DialogDescription className="sr-only">חיפוש וקישור תקדים או החלטה קשורה לתיק</DialogDescription>
         </DialogHeader>
 
         <div className="space-y-3">