perf: avoid full string scans in StrEllipsis

UTF8Length and UTF8Copy walk every byte of the input, and StrEllipsis
runs for each rendered grid cell, so large TEXT/BLOB values were fully
scanned on every paint. Exit early when the byte count already proves
there is nothing to cut, and resolve the left-hand cut position with
UTF8CodepointToByteIndex, which only walks the first MaxLen codepoints.

Output is unchanged - verified against the previous implementation with
10k randomized mixed ASCII/Korean/emoji strings plus boundary cases.
Still cuts on UTF-8 codepoint boundaries.

Author: twinn1013

source/apphelpers.pas

@@ -535,18 +535,28 @@ function Explode(Separator, Text: String): TStringList;
   @return string
 }
 function StrEllipsis(const S: String; MaxLen: Integer; FromLeft: Boolean=True): String;
+var
+  CutPos: PtrInt;
 begin
   // Truncate on UTF-8 codepoint boundaries, not raw bytes. A byte-wise cut (SetLength/Copy)
   // can split a multi-byte character and produce invalid UTF-8. On the Cocoa widgetset such a
   // string converts to a nil NSString, which crashes -[NSMenuItem initWithTitle:] when the
   // result is used as a menu caption (e.g. quick filter items).
   Result := S;
-  if UTF8Length(Result) <= MaxLen then
+  // A string with <= MaxLen bytes cannot contain more than MaxLen codepoints
+  if Length(Result) <= MaxLen then
     Exit;
-  if FromLeft then
-    Result := UTF8Copy(Result, 1, MaxLen) + '…'
-  else
+  if FromLeft then begin
+    CutPos := UTF8CodepointToByteIndex(PChar(Result), Length(Result), MaxLen);
+    if (CutPos < 0) or (CutPos >= Length(Result)) then
+      Exit;
+    SetLength(Result, CutPos);
+    Result := Result + '…';
+  end else begin
+    if UTF8Length(Result) <= MaxLen then
+      Exit;
     Result := '…' + UTF8Copy(Result, UTF8Length(Result) - MaxLen + 1, MaxLen);
+  end;
 end;