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>

Endpoints/ProfileEndpoint.cs

@@ -1,3 +1,4 @@
+using System.Linq;
 using System.Text.Json;
 using Gott.Extensions;
 using Gott.Services;
@@ -14,6 +15,24 @@ public static class ProfileEndpoint
         app.MapGet("/api/profile", HandleGet);
         app.MapPut("/api/profile", HandleUpdate);
         app.MapPut("/api/profile/password", HandlePasswordChange);
+        app.MapGet("/api/users", HandleListDirectory);
+    }
+
+    /// <summary>
+    /// Minimales User-Verzeichnis für jeden eingeloggten Nutzer (z. B. um eine Session
+    /// einem anderen Benutzer zuzuweisen). Gibt bewusst NUR id/username/displayName zurück –
+    /// keine sensiblen Felder wie defaultPath/canUseCli/isAdmin (dafür gibt es /api/admin/users).
+    /// </summary>
+    private static async System.Threading.Tasks.Task HandleListDirectory(HttpContext ctx, DatabaseService db)
+    {
+        var users = await db.GetUsers();
+        var result = users.Select(u => new
+        {
+            userId = u.UserId,
+            username = u.Username,
+            displayName = u.DisplayName
+        });
+        await ctx.Response.WriteAsJsonAsync(result);
     }
 
     private static async System.Threading.Tasks.Task HandleGet(HttpContext ctx, DatabaseService db)