Fix SVG color handling and alignment issues

- Fix hex color conversion for pyte's 256-color/truecolor format (no # prefix)
- Track column count separately from text length for proper wide char alignment
- Add tests for rgb() color format, empty rows, unicode slugify
- Improve test coverage to 80%

Bump version to 0.3.1

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "textual-webterm"
-version = "0.3.0"
+version = "0.3.1"
 description = "Serve terminal sessions over the web"
 authors = ["Will McGugan <will@textualize.io>"]
 license = "MIT"