fix(server): widen letter-spacing normalization bounds to ±256

Negative and larger letter spacing values are legitimate for extracted
text layers; clamp only outside ±256 instead of rejecting anything
below 0 (or above 80 in the extractor).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/application/design_service.go

@@ -6059,7 +6059,7 @@ func newEditableTextLayers(target design.Node, extraction design.TextExtraction)
 			lineHeight = 1.08
 		}
 		letterSpacing := layer.LetterSpacing
-		if letterSpacing < 0 {
+		if letterSpacing < -256 || letterSpacing > 256 {
 			letterSpacing = 0
 		}
 		strokeColor := strings.TrimSpace(layer.StrokeColor)