Let session owners reassign their session to another user

Previously a session could only be reassigned to another user via a
manual DB UPDATE. This adds it to the session edit UI for every owner of
a session (not just admins) - the ownership check in HandleUpdate
(existing.UserId == userId) already guarantees the caller owns the
session, so handing over one's own session is safe.

Backend:
- ProfileEndpoint: new GET /api/users returning a minimal user directory
  (userId/username/displayName only) for any logged-in user. The
  admin-gated /api/admin/users stays the source for sensitive fields
  (defaultPath/canUseCli/isAdmin).
- SessionEndpoint.HandleUpdate: validate the target user exists (400 if
  not) and pass allowUserReassign: true to UpdateSession.
- DatabaseService.UpdateSession: new allowUserReassign flag; only then is
  a non-empty userId applied as user_id (empty/null is ignored - a
  session must always have an owner).

Frontend:
- index.html: hidden #sessionOwnerRow with a user picker card in the
  session edit form (existing sessions only).
- cards.js: buildUserCard helper for the picker.
- session-page.js: initSessionUserPicker loads users via apiGetUsers(),
  preselects the current owner, and on an actual owner change asks for
  confirmation (irreversible - the session leaves the user's own list).
- api.js: apiGetUsers() -> /api/users.
- ARCHITECTURE.md: document /api/users.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

wwwroot/js/api.js

@@ -211,6 +211,9 @@ function apiShareMessage(id, includeContext) { return _post('/api/messages/' + i
 function apiGetShared(token) { return _get('/api/shared/' + token); }
 function apiDeleteShare(token) { return fetch('/api/shared/' + token, { method: 'DELETE' }); }
 
+// Minimales User-Verzeichnis (jeder eingeloggte User) – z. B. um eine Session zuzuweisen.
+function apiGetUsers() { return _get('/api/users'); }
+
 // ── Admin User API ──
 
 function apiAdminGetUsers() { return _get('/api/admin/users'); }