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/server/handlers_auth.go

@@ -18,12 +18,14 @@ import (
 
 // telegramAuthRequest carries the identity the connector extracted from a
 // validated initData payload. Username, FirstName and LanguageCode seed a
-// brand-new account's display name and language (first contact only).
+// brand-new account's display name and language; BrowserTZ (the client's detected
+// "±HH:MM" UTC offset) seeds its time zone (first contact only).
 type telegramAuthRequest struct {
 	ExternalID   string `json:"external_id"`
 	Username     string `json:"username"`
 	FirstName    string `json:"first_name"`
 	LanguageCode string `json:"language_code"`
+	BrowserTZ    string `json:"browser_tz"`
 }
 
 // handleTelegramAuth provisions (or finds) the account bound to a Telegram
@@ -35,7 +37,7 @@ func (s *Server) handleTelegramAuth(c *gin.Context) {
 		abortBadRequest(c, "external_id is required")
 		return
 	}
-	acc, created, err := s.accounts.ProvisionTelegram(c.Request.Context(), req.ExternalID, req.LanguageCode, req.Username, req.FirstName)
+	acc, created, err := s.accounts.ProvisionTelegram(c.Request.Context(), req.ExternalID, req.LanguageCode, req.Username, req.FirstName, req.BrowserTZ)
 	if err != nil {
 		s.abortErr(c, err)
 		return
@@ -97,9 +99,21 @@ func (s *Server) handlePushTarget(c *gin.Context) {
 	})
 }
 
-// handleGuestAuth provisions a fresh ephemeral guest account and mints a session.
+// guestAuthRequest carries the guest bootstrap's optional time-zone seed: BrowserTZ
+// (the client's detected "±HH:MM" UTC offset) is written to the new guest account's
+// time zone, so robot timing is anchored to the player's zone from the first game.
+type guestAuthRequest struct {
+	BrowserTZ string `json:"browser_tz"`
+}
+
+// handleGuestAuth provisions a fresh ephemeral guest account and mints a session,
+// seeding its time zone from the optional detected browser offset.
 func (s *Server) handleGuestAuth(c *gin.Context) {
-	acc, err := s.accounts.ProvisionGuest(c.Request.Context())
+	// The body is optional: an absent or malformed one simply yields no time-zone seed
+	// (the account keeps the UTC default), so a bind error must not fail the bootstrap.
+	var req guestAuthRequest
+	_ = c.ShouldBindJSON(&req)
+	acc, err := s.accounts.ProvisionGuest(c.Request.Context(), req.BrowserTZ)
 	if err != nil {
 		s.abortErr(c, err)
 		return
@@ -107,9 +121,12 @@ func (s *Server) handleGuestAuth(c *gin.Context) {
 	s.mintSession(c, acc)
 }
 
-// emailRequest is an email-login code request.
+// emailRequest is an email-login code request. BrowserTZ (the client's detected
+// "±HH:MM" UTC offset) seeds the time zone of an account provisioned here on first
+// contact (the email account is created at the request step, not at login).
 type emailRequest struct {
-	Email string `json:"email"`
+	Email     string `json:"email"`
+	BrowserTZ string `json:"browser_tz"`
 }
 
 // handleEmailRequest issues a login confirm-code to the email. It always reports
@@ -121,7 +138,7 @@ func (s *Server) handleEmailRequest(c *gin.Context) {
 		abortBadRequest(c, "email is required")
 		return
 	}
-	if _, err := s.emails.RequestLoginCode(c.Request.Context(), req.Email); err != nil {
+	if _, err := s.emails.RequestLoginCode(c.Request.Context(), req.Email, req.BrowserTZ); err != nil {
 		s.abortErr(c, err)
 		return
 	}