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- Schema version: outdated databases are wiped and recreated on start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/App.tsx

@@ -32,6 +32,7 @@ import {
   triggerHaptic,
   connectWebSocket,
   disconnectWebSocket,
+  isSharingDisabled,
 } from './lib/api';
 import {
   AVAILABLE_THEMES,
@@ -273,12 +274,16 @@ export const App: React.FC = () => {
               badge: stats.listsCount,
               onClick: () => setActiveTab('lists'),
             },
-            {
-              id: 'goto-groups',
-              label: 'Teams & Gruppen',
-              icon: <Users className="w-4 h-4" />,
-              onClick: () => setActiveTab('groups'),
-            },
+            ...(isSharingDisabled()
+              ? []
+              : [
+                  {
+                    id: 'goto-groups',
+                    label: 'Gruppen',
+                    icon: <Users className="w-4 h-4" />,
+                    onClick: () => setActiveTab('groups'),
+                  },
+                ]),
           ],
         },
       ]);
@@ -618,7 +623,7 @@ export const App: React.FC = () => {
     tasks: 'Aufgaben',
     workspace: 'Workspace',
     lists: 'Listen',
-    groups: 'Teams & Gruppen',
+    groups: 'Gruppen',
     user_settings: 'Einstellungen',
     admin_settings: 'Administration',
     help: 'Hilfe',
@@ -802,6 +807,7 @@ export const App: React.FC = () => {
                     <span>Kontoeinstellungen</span>
                   </button>
 
+                  {!isSharingDisabled() && (
                   <button
                     onClick={() => {
                       triggerHaptic(10);
@@ -815,6 +821,7 @@ export const App: React.FC = () => {
                     <Users className="w-4 h-4 text-accent" />
                     <span>Gruppen</span>
                   </button>
+                  )}
 
                   <button
                     onClick={handleOpenHelp}
@@ -1367,6 +1374,7 @@ export const App: React.FC = () => {
                 <span>Kontoeinstellungen</span>
               </button>
 
+              {!isSharingDisabled() && (
               <button
                 type="button"
                 onClick={() => {
@@ -1381,6 +1389,7 @@ export const App: React.FC = () => {
                 <Users className="w-4 h-4 text-accent" />
                 <span>Gruppen</span>
               </button>
+              )}
 
               <button
                 type="button"