Fix cursor/background vertical alignment in SVG screenshots

- Remove dominant-baseline: text-before-edge (has Safari compatibility issues)
- Use separate y positions for rect (top of cell) and text (baseline)
- rect_y = padding + row * line_height (top of cell)
- text_y = rect_y + font_size (alphabetic baseline position)

This ensures background rects and text are properly aligned across all
browsers, fixing the half-line vertical offset on cursor blocks.

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "textual-webterm"
-version = "0.3.11"
+version = "0.3.12"
 description = "Serve terminal sessions over the web"
 authors = ["Will McGugan <will@textualize.io>"]
 license = "MIT"