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/cases/drafts-panel.tsx

@@ -8,6 +8,7 @@ import { Textarea } from "@/components/ui/textarea";
 import {
   Dialog,
   DialogContent,
+  DialogDescription,
   DialogHeader,
   DialogTitle,
   DialogTrigger,
@@ -323,6 +324,7 @@ export function DraftsPanel({
           <DialogContent className="sm:max-w-sm" dir="rtl">
             <DialogHeader>
               <DialogTitle>מחיקת טיוטה</DialogTitle>
+              <DialogDescription className="sr-only">אישור מחיקת קובץ הטיוטה</DialogDescription>
             </DialogHeader>
             <p className="text-sm text-ink-muted">
               למחוק את הקובץ{" "}
@@ -493,6 +495,7 @@ function NewCaseFeedbackDialog({ caseNumber }: { caseNumber: string }) {
       <DialogContent className="sm:max-w-lg" dir="rtl">
         <DialogHeader>
           <DialogTitle>הערת יו״ר — תיק {caseNumber}</DialogTitle>
+          <DialogDescription className="sr-only">הוספת הערת יו״ר על בלוק בהחלטה</DialogDescription>
         </DialogHeader>
         <form onSubmit={handleSubmit} className="space-y-4 mt-2">
           <div className="grid grid-cols-2 gap-3">