fix(auth): new users inherit the system-wide default timezone

default_timezone existed as a system setting (admin-editable, exposed
via SettingsRegistry.DefaultTimezone) but nothing read it at user
creation — the CreateUser query didn't even carry the column, so every
new user got the DB default UTC regardless. Both creation paths
(public register + admin create) now seed the user's timezone from the
setting, validated with time.LoadLocation and falling back to UTC when
unwired/empty/invalid.

Test-infra fixes surfaced by the run: TestUpdateProfileSelfEdit was
self-editing the SHARED admin (renaming it past the harness cleanup's
preserve-list, leaking a second admin row that broke the last-admin
tests) — it now edits a dedicated user with a unique name; the
last-admin tests get an ensureSingleAdmin precondition helper; the
websocket collection name is run-unique (non-test names persist in the
shared DB and UNIQUE(user_id,name) 409s on rerun).

cmd/server/tests/ownership_and_admin_test.go

@@ -190,6 +190,21 @@ func (s *OwnershipAndAdminTestSuite) TestAdminCreateUser() {
 	assert.Equal(s.T(), unique, created["username"])
 	assert.Equal(s.T(), "user", created["role"])
 
+	// New users inherit the system-wide default timezone (this was the
+	// point of the setting; before 2026-10-13 every new user got UTC
+	// regardless of it).
+	expectedTZ := "UTC"
+	var settingTZ string
+	err = s.setup.DBPool.QueryRow(s.setup.QueueCtx,
+		"SELECT setting_value FROM system_settings WHERE setting_key = 'default_timezone'").Scan(&settingTZ)
+	if err == nil {
+		if _, locErr := time.LoadLocation(settingTZ); locErr == nil && settingTZ != "" {
+			expectedTZ = settingTZ
+		}
+	}
+	assert.Equal(s.T(), expectedTZ, created["timezone"],
+		"created user must carry the system default timezone")
+
 	// Duplicate username → 409.
 	req, _ = http.NewRequest("POST", s.setup.Server.URL+"/api/auth/users", bytes.NewBuffer(payload))
 	req.Header.Set("Content-Type", "application/json")