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/sync_integration_test.go

@@ -55,6 +55,7 @@ func createSyncTestUser(t *testing.T, db *database.Queries) pgtype.UUID {
 		FirstName:    pgtype.Text{String: "Test", Valid: true},
 		LastName:     pgtype.Text{String: "Sync", Valid: true},
 		Role:         "user",
+		Timezone:     pgtype.Text{String: "UTC", Valid: true},
 	})
 	require.NoError(t, err)