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- Schema version: outdated databases are wiped and recreated on start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

@@ -1398,7 +1398,7 @@ export const TasksView: React.FC<TasksViewProps> = ({
                   <span className="ml-1 badge-count">{tl.tasksCount}</span>
                 )}
                 {(() => {
-                  const isShared = (tl.shares && tl.shares.length > 0) || Boolean(tl.teamId) || (tl.members && tl.members.length > 1);
+                  const isShared = (tl.shares && tl.shares.length > 0) || (tl.members && tl.members.length > 1);
                   if (!isShared) return null;
                   const otherMembers = (tl.members || []).filter((m) => m.userId !== currentUser?.id);
                   if (otherMembers.length > 0) {
@@ -1448,7 +1448,7 @@ export const TasksView: React.FC<TasksViewProps> = ({
             const sharesCount = (currentSelectedList.shares?.length || 0) > 0
               ? currentSelectedList.shares.length
               : (currentSelectedList.members?.filter((m) => m.userId !== listOwnerId).length || 0);
-            const isShared = sharesCount > 0 || Boolean(currentSelectedList.teamId) || ((currentSelectedList.members?.length || 0) > 1);
+            const isShared = sharesCount > 0 || ((currentSelectedList.members?.length || 0) > 1);
             const ownerMember = currentSelectedList.members?.find((m) => m.role === 'owner' || m.userId === listOwnerId);
             const ownerDisplayName = currentSelectedList.ownerName || ownerMember?.displayName || ownerMember?.email || 'Eigentümer';