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/documents-panel.tsx

@@ -6,6 +6,7 @@ import { Progress } from "@/components/ui/progress";
 import {
   Dialog,
   DialogContent,
+  DialogDescription,
   DialogHeader,
   DialogTitle,
   DialogFooter,
@@ -127,6 +128,7 @@ function DocumentPreviewDialog({
       <DialogContent className="sm:max-w-2xl max-h-[80vh] flex flex-col" dir="rtl">
         <DialogHeader>
           <DialogTitle className="text-right">{displayName}</DialogTitle>
+          <DialogDescription className="sr-only">תצוגה מקדימה של תוכן המסמך</DialogDescription>
         </DialogHeader>
         <div className="flex-1 overflow-hidden">
           {loading && (
@@ -184,6 +186,7 @@ function DeleteConfirmDialog({
       <DialogContent dir="rtl">
         <DialogHeader>
           <DialogTitle className="text-right">מחיקת מסמך</DialogTitle>
+          <DialogDescription className="sr-only">אישור מחיקת המסמך מהתיק</DialogDescription>
         </DialogHeader>
         <p className="text-sm text-ink-muted text-right">
           האם למחוק את המסמך <strong>&ldquo;{displayName}&rdquo;</strong>?