Fix hex color handling in SVG exporter

pyte provides 256-color/truecolor values without # prefix (e.g., 'ff8700').
Added check to prepend # for 6-digit hex strings.

src/textual_webterm/svg_exporter.py

@@ -68,10 +68,15 @@ def _color_to_hex(color: str, is_foreground: bool = True) -> str:
     if color == "default":
         return DEFAULT_FG if is_foreground else DEFAULT_BG
 
-    # Already a hex color
+    # Already a hex color with #
     if color.startswith("#"):
         return color
 
+    # Hex color without # prefix (pyte's 256-color/truecolor format)
+    # Check if it looks like a hex color (6 hex digits)
+    if len(color) == 6 and all(c in "0123456789abcdefABCDEF" for c in color):
+        return f"#{color}"
+
     # Named color lookup (case-insensitive)
     lower = color.lower()
     if lower in ANSI_COLORS: