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- Schema version: outdated databases are wiped and recreated on start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/features/teams/GroupsView.tsx

@@ -13,7 +13,7 @@ import {
   LogOut,
 } from 'lucide-react';
 import { Team, User } from '../../lib/types';
-import { api, triggerHaptic } from '../../lib/api';
+import { api, isSharingDisabled, triggerHaptic } from '../../lib/api';
 import { UserAutocomplete } from '../UserAutocomplete';
 import { UserAvatar } from '../../components/UserAvatar';
 
@@ -187,6 +187,18 @@ export const GroupsView: React.FC<GroupsViewProps> = ({ currentUser, onBack }) =
       !managedTeams.some((mt) => mt.id === t.id)
   );
 
+  if (isSharingDisabled()) {
+    return (
+      <div className="w-full space-y-3">
+        <button onClick={onBack} className="btn-action h-8 px-2.5 text-xs flex items-center gap-1.5 cursor-pointer">
+          <ArrowLeft className="w-3.5 h-3.5" />
+          <span>Zurück</span>
+        </button>
+        <p className="text-xs text-muted">Gruppen sind auf diesem Server deaktiviert (Sichtbarkeits-Modus „Privat“).</p>
+      </div>
+    );
+  }
+
   return (
     <div className="w-full space-y-5">
       {/* Page Header */}
@@ -203,13 +215,13 @@ export const GroupsView: React.FC<GroupsViewProps> = ({ currentUser, onBack }) =
           <div>
             <h1 className="text-base font-bold text-main tracking-tight flex items-center gap-2">
               <Users className="w-4 h-4 text-accent" />
-              <span>Gruppen & Teams</span>
+              <span>Gruppen</span>
               <span className="text-xs px-2 py-0.5 rounded-full bg-panel-elevated text-muted border border-border">
                 {teams.length}
               </span>
             </h1>
             <p className="text-xs text-muted mt-0.5">
-              Erstelle und verwalte Gruppen (z. B. Familie, WG oder Projektteams) für gemeinsame Notizen, Aufgaben und Listen.
+              Erstelle Gruppen (z. B. Familie, WG oder Projektteam) aus Personen, die du sehen kannst, und teile Notizen, Aufgaben und Listen mit allen auf einmal.
             </p>
           </div>
         </div>