fix(account): Telegram display-name falls back to the @username verbatim

When the Telegram first name yields no usable letters, fall back to the @username
taken whole (trimmed + length-capped, never character-stripped like the real name)
rather than a sanitized form; the generated placeholder is reached only when no
username is set. Precedence: real name -> @username (verbatim) -> placeholder.

backend/internal/account/account.go

@@ -207,10 +207,11 @@ type provisionSeed struct {
 
 // telegramSeed derives the create-time seed from Telegram launch fields: a
 // supported preferred language from languageCode (an ISO-639 code, possibly
-// region-tagged like "ru-RU"), and a display name sanitized from firstName or,
-// failing that, username (sanitizeDisplayName strips disallowed characters to the
-// editable format). When neither yields any letters, it falls back to a generated
-// placeholder in the seeded language (placeholderDisplayName).
+// region-tagged like "ru-RU"), and a display name. The name precedence is the real
+// name (firstName, sanitized to the editable format) → the @username taken verbatim
+// (already a valid handle, only trimmed and length-capped, never character-stripped)
+// → a generated placeholder in the seeded language (placeholderDisplayName), reached
+// only when firstName has no usable letters and no username is set.
 func telegramSeed(languageCode, username, firstName string) provisionSeed {
 	var seed provisionSeed
 	if lang, _, _ := strings.Cut(strings.ToLower(strings.TrimSpace(languageCode)), "-"); lang == "en" || lang == "ru" {
@@ -218,7 +219,13 @@ func telegramSeed(languageCode, username, firstName string) provisionSeed {
 	}
 	name := sanitizeDisplayName(firstName)
 	if name == "" {
-		name = sanitizeDisplayName(username)
+		// The real name yielded nothing usable: fall back to the @username verbatim
+		// (Telegram guarantees a valid handle), only trimmed and capped to the column
+		// width — never character-stripped like the real name.
+		name = strings.TrimSpace(username)
+		if r := []rune(name); len(r) > maxDisplayName {
+			name = strings.TrimRight(string(r[:maxDisplayName]), " ")
+		}
 	}
 	if name == "" {
 		name = placeholderDisplayName(seed.preferredLanguage)