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/inttest/account_test.go

@@ -110,15 +110,15 @@ func identityConfirmed(t *testing.T, kind, externalID string) bool {
 }
 
 // TestProvisionTelegramSeedsNewAccountOnly checks that Telegram first contact
-// seeds the new account's language and display name from the launch fields,
-// defaults the in-app-only flag on, and never overwrites an existing account on a
-// later login (language seeding).
+// seeds the new account's language, display name and time zone from the launch
+// fields / detected offset, defaults the in-app-only flag on, and never overwrites
+// an existing account on a later login (language and zone seeding).
 func TestProvisionTelegramSeedsNewAccountOnly(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
 	ext := "tg-" + uuid.NewString()
 
-	acc, created, err := store.ProvisionTelegram(ctx, ext, "ru-RU", "thehandle", "Иван")
+	acc, created, err := store.ProvisionTelegram(ctx, ext, "ru-RU", "thehandle", "Иван", "+03:00")
 	if err != nil {
 		t.Fatalf("provision telegram: %v", err)
 	}
@@ -131,12 +131,15 @@ func TestProvisionTelegramSeedsNewAccountOnly(t *testing.T) {
 	if acc.DisplayName != "Иван" {
 		t.Errorf("DisplayName = %q, want Иван", acc.DisplayName)
 	}
+	if acc.TimeZone != "+03:00" {
+		t.Errorf("TimeZone = %q, want the seeded +03:00", acc.TimeZone)
+	}
 	if !acc.NotificationsInAppOnly {
 		t.Error("NotificationsInAppOnly should default to true")
 	}
 
 	// A later login with different fields returns the same account, unchanged.
-	again, created, err := store.ProvisionTelegram(ctx, ext, "en", "other", "Other")
+	again, created, err := store.ProvisionTelegram(ctx, ext, "en", "other", "Other", "+09:00")
 	if err != nil {
 		t.Fatalf("re-provision telegram: %v", err)
 	}
@@ -146,8 +149,53 @@ func TestProvisionTelegramSeedsNewAccountOnly(t *testing.T) {
 	if again.ID != acc.ID {
 		t.Errorf("re-provision id = %s, want %s", again.ID, acc.ID)
 	}
-	if again.PreferredLanguage != "ru" || again.DisplayName != "Иван" {
-		t.Errorf("existing account overwritten: lang=%q name=%q", again.PreferredLanguage, again.DisplayName)
+	if again.PreferredLanguage != "ru" || again.DisplayName != "Иван" || again.TimeZone != "+03:00" {
+		t.Errorf("existing account overwritten: lang=%q name=%q tz=%q", again.PreferredLanguage, again.DisplayName, again.TimeZone)
+	}
+}
+
+// TestProvisionSeedsTimeZone checks the create-time time-zone seed across paths: a
+// valid detected offset is stored verbatim (even "+00:00", which is deliberately
+// distinct from the unset "UTC" default), a guest is seeded the same way, and a
+// missing or malformed offset falls back to the "UTC" column default rather than
+// being guessed at.
+func TestProvisionSeedsTimeZone(t *testing.T) {
+	ctx := context.Background()
+	store := account.NewStore(testDB)
+
+	// A detected zero offset is written as "+00:00" — we record that the zone was
+	// detected (and equals UTC), distinct from the "UTC" default meaning "unknown".
+	utcDetected, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Zero", "+00:00")
+	if err != nil {
+		t.Fatalf("provision telegram +00:00: %v", err)
+	}
+	if utcDetected.TimeZone != "+00:00" {
+		t.Errorf("TimeZone = %q, want the seeded +00:00", utcDetected.TimeZone)
+	}
+
+	// A malformed offset is dropped: the account keeps the UTC default.
+	bad, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Bad", "not-a-zone")
+	if err != nil {
+		t.Fatalf("provision telegram bad tz: %v", err)
+	}
+	if bad.TimeZone != "UTC" {
+		t.Errorf("TimeZone = %q, want UTC fallback for a malformed offset", bad.TimeZone)
+	}
+
+	// A guest is seeded its detected offset; an empty one keeps the UTC default.
+	guest, err := store.ProvisionGuest(ctx, "-05:30")
+	if err != nil {
+		t.Fatalf("provision guest: %v", err)
+	}
+	if guest.TimeZone != "-05:30" {
+		t.Errorf("guest TimeZone = %q, want the seeded -05:30", guest.TimeZone)
+	}
+	plainGuest, err := store.ProvisionGuest(ctx, "")
+	if err != nil {
+		t.Fatalf("provision plain guest: %v", err)
+	}
+	if plainGuest.TimeZone != "UTC" {
+		t.Errorf("plain guest TimeZone = %q, want UTC default", plainGuest.TimeZone)
 	}
 }
 
@@ -156,7 +204,7 @@ func TestProvisionTelegramSeedsNewAccountOnly(t *testing.T) {
 // language CHECK.
 func TestProvisionTelegramUnknownLanguageDefaults(t *testing.T) {
 	ctx := context.Background()
-	acc, _, err := account.NewStore(testDB).ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "fr", "", "")
+	acc, _, err := account.NewStore(testDB).ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "fr", "", "", "")
 	if err != nil {
 		t.Fatalf("provision telegram: %v", err)
 	}
@@ -172,7 +220,7 @@ func TestProvisionTelegramUnknownLanguageDefaults(t *testing.T) {
 func TestHighRateFlagRoundTrip(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
-	acc, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Player")
+	acc, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Player", "")
 	if err != nil {
 		t.Fatalf("provision telegram: %v", err)
 	}
@@ -228,7 +276,7 @@ func TestIdentityExternalID(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
 	ext := "tg-" + uuid.NewString()
-	acc, _, err := store.ProvisionTelegram(ctx, ext, "en", "", "Tg User")
+	acc, _, err := store.ProvisionTelegram(ctx, ext, "en", "", "Tg User", "")
 	if err != nil {
 		t.Fatalf("provision telegram: %v", err)
 	}
@@ -253,7 +301,7 @@ func TestIdentityExternalID(t *testing.T) {
 func TestNotificationsInAppOnlyRoundTrip(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
-	acc, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Player")
+	acc, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Player", "")
 	if err != nil {
 		t.Fatalf("provision telegram: %v", err)
 	}