fix(account): seed the email account display name from the local part

An email account was provisioned with no display name (unlike Telegram/VK, which
seed one), so an email login showed an empty name. Seed it from the email's local
part (before '@', trimmed and capped to the column width) on first contact; the
user can rename it later. Only new accounts are seeded — an existing account's name
is never overwritten.

backend/internal/account/account.go

@@ -122,21 +122,35 @@ func (s *Store) ProvisionByIdentity(ctx context.Context, kind, externalID string
 
 // ProvisionEmail returns the account owning the email identity externalID, creating
 // it on first contact with browserTZ — the client's detected "±HH:MM" UTC offset —
-// seeded into its time zone and language seeded from the client's UI language. Like
+// seeded into its time zone, language seeded from the client's UI language, and its
+// display name seeded from the email's local part (so it is not left nameless). Like
 // ProvisionByIdentity it is race-safe and leaves an existing account untouched, so a
-// returning user's saved zone and language are never overwritten. The email account is
+// returning user's saved zone, language and name are never overwritten. The email account is
 // created here (the code-request step), not at the later login, so this is where its
 // zone and language are seeded. It is created flagged is_guest with an unconfirmed
 // email identity: an abandoned, never-confirmed login is then reaped like any guest,
 // freeing the reserved address, and confirming the code clears the guest flag.
 func (s *Store) ProvisionEmail(ctx context.Context, externalID, browserTZ, language string) (Account, error) {
 	return s.provision(ctx, KindEmail, externalID, provisionSeed{
+		displayName:       emailDisplayName(externalID),
 		timeZone:          seedZone(browserTZ),
 		preferredLanguage: supportedLanguage(language),
 		isGuest:           true,
 	})
 }
 
+// emailDisplayName derives a display name from an email address — the local part
+// before '@', trimmed and capped to the column width — so a new email account is not
+// left nameless. It is only the first-contact seed; the user can rename it later.
+func emailDisplayName(email string) string {
+	local, _, _ := strings.Cut(email, "@")
+	local = strings.TrimSpace(local)
+	if r := []rune(local); len(r) > maxDisplayName {
+		local = strings.TrimRight(string(r[:maxDisplayName]), " ")
+	}
+	return local
+}
+
 // supportedLanguage returns code normalised to a supported UI language ("en" or
 // "ru"), or "" when it maps to neither, so a new account keeps the 'en' default. It
 // accepts region-tagged codes ("ru-RU").

backend/internal/inttest/email_test.go

@@ -373,3 +373,24 @@ func TestConfirmByTokenLinkMerge(t *testing.T) {
 		t.Fatalf("merge result = %+v, want NeedsMerge with owner=%s", res, owner)
 	}
 }
+
+// TestEmailAccountSeedsDisplayName seeds a new email account's display name from the
+// email's local part, so an email login is not left nameless.
+func TestEmailAccountSeedsDisplayName(t *testing.T) {
+	ctx := context.Background()
+	store := account.NewStore(testDB)
+	svc := account.NewEmailService(store, &capturingMailer{}, "https://erudit-game.ru")
+	local := "kaya-" + uuid.NewString()[:8]
+
+	id, err := svc.RequestLoginCode(ctx, local+"@example.com", "", "en")
+	if err != nil {
+		t.Fatalf("request login: %v", err)
+	}
+	acc, err := store.GetByID(ctx, id)
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if acc.DisplayName != local {
+		t.Errorf("display name = %q, want the email local part %q", acc.DisplayName, local)
+	}
+}