feat: Kanban touch drag, admins delete backups, bigger share message

- Kanban on phones/tablets: hold a card, drag it onto a card or column, release; the page scrolls at the edges
- Admin settings: database backups can be deleted (DELETE /api/v1/admin/backups/{name})
- Share message: large Liste/Aufgabe buttons, close button, tap outside closes

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/settings/BackupAdminCard.tsx

@@ -1,5 +1,5 @@
 import React, { useEffect, useState } from 'react';
-import { Archive, Download, Loader2 } from 'lucide-react';
+import { Archive, Download, Loader2, Trash2 } from 'lucide-react';
 import { BackupFile, SystemSettings } from '../../lib/types';
 import { api } from '../../lib/api';
 import { ToggleSwitch } from '../../components/ToggleSwitch';
@@ -41,6 +41,16 @@ export const BackupAdminCard: React.FC<BackupAdminCardProps> = ({ settings, onSa
     }
   };
 
+  const remove = async (name: string) => {
+    if (!window.confirm(`Backup ${name} endgültig löschen?`)) return;
+    setError(null);
+    try {
+      setFiles(await api.adminDeleteBackup(name));
+    } catch (err: any) {
+      setError(err.message || 'Löschen fehlgeschlagen');
+    }
+  };
+
   return (
     <div className="p-4 bg-canvas border border-border rounded-card space-y-3">
       <div className="flex items-center justify-between gap-4">
@@ -109,14 +119,25 @@ export const BackupAdminCard: React.FC<BackupAdminCardProps> = ({ settings, onSa
                   {new Date(f.createdAt).toLocaleString('de-DE')} · {formatBytes(f.size)}
                 </p>
               </div>
-              <button
-                type="button"
-                onClick={() => download(f.name)}
-                className="btn-action h-11 sm:h-7 px-2.5 text-[11px] inline-flex items-center gap-1 shrink-0"
-              >
-                <Download className="w-3.5 h-3.5" />
-                Download
-              </button>
+              <div className="flex items-center gap-1.5 shrink-0">
+                <button
+                  type="button"
+                  onClick={() => download(f.name)}
+                  className="btn-action h-11 sm:h-7 px-2.5 text-[11px] inline-flex items-center gap-1"
+                >
+                  <Download className="w-3.5 h-3.5" />
+                  Download
+                </button>
+                <button
+                  type="button"
+                  onClick={() => remove(f.name)}
+                  title="Backup löschen"
+                  aria-label={`Backup ${f.name} löschen`}
+                  className="btn-action h-11 w-11 sm:h-7 sm:w-7 inline-flex items-center justify-center text-faint hover:text-status-err"
+                >
+                  <Trash2 className="w-3.5 h-3.5" />
+                </button>
+              </div>
             </div>
           ))
         )}