feat: organizations decide who sees whom; sharing only by owners

- Server admins manage organizations (one per user); new admin section,
  organization on user creation and invites, setup creates the first one
- Sharing modes private / organizations / open, enforced centrally
  (CanSee, CanShareWith) for directory, all shares and groups; unknown and
  invisible targets give the same error
- Losing visibility (org change, mode change, leaving a group) prunes shares,
  group memberships and task assignments; dry run warns before tightening
- Only owners share; editors can no longer reshare lists or remove others'
  shares; legacy task_lists.team_id and old /organizations endpoints removed
  (FLAW-06, FLAW-10, FLAW-24)
- Schema version: outdated databases are wiped and recreated on start
- Fix setup sample data (createSampleData was ignored); setup and admin
  use a shared ToggleSwitch instead of checkboxes
- Docs: sharing-and-permissions rewritten, ui-patterns, REST, DB, flaws

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

frontend/src/components/UnifiedShareModal.tsx

@@ -11,6 +11,7 @@ import {
 import { User, Team, UnifiedShare } from '../lib/types';
 import { UserAutocomplete } from '../features/UserAutocomplete';
 import { UserAvatar } from './UserAvatar';
+import { isSharingDisabled } from '../lib/api';
 
 export interface UnifiedShareModalProps {
   isOpen: boolean;
@@ -147,7 +148,11 @@ export const UnifiedShareModal: React.FC<UnifiedShareModalProps> = ({
         )}
 
         {/* Add Share Section (Only if owner / manager) */}
-        {isOwner ? (
+        {isSharingDisabled() ? (
+          <div className="p-3 bg-panel-elevated border border-border rounded-card text-xs text-muted">
+            Teilen ist auf diesem Server deaktiviert (Sichtbarkeits-Modus „Privat“).
+          </div>
+        ) : isOwner ? (
           <form onSubmit={handleAdd} className="space-y-3 p-3 bg-panel-elevated border border-border rounded-card">
             <div className="flex items-center justify-between gap-2">
               <span className="text-xs font-semibold text-main">Freigabe hinzufügen:</span>