feat(account): seed the time zone from the client's detected offset at creation

A new account's time_zone defaulted to 'UTC' until the player saved a profile, so the
robot's sleep window and the turn-timeout away-window sweeper — both anchored to the
account zone via account.ResolveZone — ran on UTC for every fresh player, skewing
robot-game timing until a manual Settings save. Seed the zone at creation instead, from
the client's detected "±HH:MM" offset.

- Carry browser_tz on the three account-creating auth requests (TelegramLoginRequest,
  GuestLoginRequest, EmailRequestRequest — the email account is provisioned at the
  code-request step, not at login) through the fbs envelope (+ Go/TS codegen), the
  gateway transcode + backend client, and the backend auth handlers into
  ProvisionTelegram / ProvisionGuest / ProvisionEmail.
- create() now writes time_zone explicitly: the validated detected offset, or 'UTC'
  (equal to the column default) when absent or malformed — deterministic, never guessed.
  The column is already NOT NULL DEFAULT 'UTC', so no migration is needed and existing
  accounts keep 'UTC'. An existing account is never overwritten on re-login.
- A detected zero offset is stored as "+00:00" (the zone is known and equals UTC),
  distinct from the "UTC" default that means "unknown" — which the feedback console's
  three-zone Filed display already reflects.
- Guard the guest handler against an empty payload (the bootstrap historically carried
  none) so it degrades to no-seed rather than panicking in GetRootAs*.
- Tests: zone seeding across Telegram/guest/email plus the "+00:00"/malformed/empty
  cases and the not-overwrite rule; codec round-trip for the three auth encoders.
  ARCHITECTURE + FUNCTIONAL(+ru) updated.

pkg/fbs/scrabblefb/GuestLoginRequest.go

@@ -49,12 +49,23 @@ func (rcv *GuestLoginRequest) Locale() []byte {
 	return nil
 }
 
+func (rcv *GuestLoginRequest) BrowserTz() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
 func GuestLoginRequestStart(builder *flatbuffers.Builder) {
-	builder.StartObject(1)
+	builder.StartObject(2)
 }
 func GuestLoginRequestAddLocale(builder *flatbuffers.Builder, locale flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(locale), 0)
 }
+func GuestLoginRequestAddBrowserTz(builder *flatbuffers.Builder, browserTz flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(browserTz), 0)
+}
 func GuestLoginRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }