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- Schema version: outdated databases are wiped and recreated on start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>

Doku/ui-patterns.md

@@ -32,6 +32,13 @@ Komponente: [AvatarStack.tsx](../frontend/src/components/AvatarStack.tsx) (nutzt
 | Aufgabenzeile und Kanban-Karte | zugewiesene Personen (nur in geteilten Listen) |
 | Eintrag einer geteilten Liste | wer ihn hinzugefügt hat (`createdBy`); ältere Einträge ohne Autor zeigen nichts |
 
+## Schalter
+
+Komponente: [ToggleSwitch.tsx](../frontend/src/components/ToggleSwitch.tsx)
+
+- Ein/Aus-Einstellungen sind immer Schalter, keine Checkbox-Haken (Einrichtung, Admin-Bereich).
+- Text links (Titel, darunter eine kurze Erklärung in `text-muted`), Schalter rechts.
+
 ## Zähler
 
 CSS-Klasse `badge-count` in [index.css](../frontend/src/index.css): runder Zähler in der Form der Zähler der Navigationsleiste, aber **monochrom** (grau gefüllt) statt in der Akzentfarbe.