/mybots: Edit Bot summary screen, return-to-menu, clickable @mentions

- Edit Bot now shows the current value of every field (Name/About/
  Description/Botpic/Commands) like BotFather, with real botpic status
  via a new PeerHasAvatar port method.
- After editing a field the dialog lands back on a fresh Edit Bot menu
  (working "Back to bot" / "Bots list" buttons) instead of ending, so a
  follow-up button press no longer reports the button as expired.
- Service-bot messages now render @username as a tappable mention entity.

internal/app/bots/service_bot_entities.go

@@ -45,6 +45,10 @@ func serviceBotReplyEntities(text string, explicit []domain.MessageEntity) []dom
 		offset, length := utf16Range(text, span.start, span.end)
 		appendEntity(domain.MessageEntity{Type: domain.MessageEntityBotCommand, Offset: offset, Length: length})
 	}
+	for _, span := range serviceBotMentionByteSpans(text) {
+		offset, length := utf16Range(text, span.start, span.end)
+		appendEntity(domain.MessageEntity{Type: domain.MessageEntityMention, Offset: offset, Length: length})
+	}
 	sort.SliceStable(out, func(i, j int) bool {
 		if out[i].Offset != out[j].Offset {
 			return out[i].Offset < out[j].Offset
@@ -138,6 +142,42 @@ func serviceBotCommandByteSpans(text string) []serviceBotEntitySpan {
 	return spans
 }
 
+// serviceBotMentionByteSpans finds "@username" runs so a service bot's messages
+// render the mention as a tappable link. A username is 5-32 of [A-Za-z0-9_]; the
+// "@" must not sit right after a word character (so "you@example" is not a
+// mention) and the run must not be followed by one.
+func serviceBotMentionByteSpans(text string) []serviceBotEntitySpan {
+	var spans []serviceBotEntitySpan
+	for i := 0; i < len(text); {
+		r, size := utf8.DecodeRuneInString(text[i:])
+		if r != '@' || !serviceBotCommandStart(text, i) {
+			i += size
+			continue
+		}
+		start := i
+		i += size
+		nameStart := i
+		for i < len(text) {
+			c, csize := utf8.DecodeRuneInString(text[i:])
+			if !serviceBotCommandChar(c) {
+				break
+			}
+			i += csize
+		}
+		nameLen := i - nameStart
+		if nameLen < 5 || nameLen > 32 {
+			continue
+		}
+		if i < len(text) {
+			if next, _ := utf8.DecodeRuneInString(text[i:]); serviceBotCommandChar(next) {
+				continue
+			}
+		}
+		spans = append(spans, serviceBotEntitySpan{start: start, end: i})
+	}
+	return spans
+}
+
 func serviceBotCommandStart(text string, byteIndex int) bool {
 	if byteIndex == 0 {
 		return true