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.

backend/internal/account/email.go

@@ -131,13 +131,15 @@ func (s *EmailService) ConfirmCode(ctx context.Context, accountID uuid.UUID, ema
 // the unauthenticated email-login entry point and, unlike RequestCode,
 // does not refuse an already-confirmed email — that is the ordinary returning-user
 // login. The code is mailed to the address, so only its real owner can complete
-// the login. It returns the target account id for the subsequent LoginWithCode.
-func (s *EmailService) RequestLoginCode(ctx context.Context, email string) (uuid.UUID, error) {
+// the login. On first contact browserTZ (the client's detected "±HH:MM" UTC offset)
+// seeds the new account's time zone. It returns the target account id for the
+// subsequent LoginWithCode.
+func (s *EmailService) RequestLoginCode(ctx context.Context, email, browserTZ string) (uuid.UUID, error) {
 	addr, err := normalizeEmail(email)
 	if err != nil {
 		return uuid.UUID{}, err
 	}
-	acc, err := s.store.ProvisionByIdentity(ctx, KindEmail, addr)
+	acc, err := s.store.ProvisionEmail(ctx, addr, browserTZ)
 	if err != nil {
 		return uuid.UUID{}, err
 	}