Scale box-drawing characters vertically to fill line height

Box-drawing characters (│┃║┌┐└┘├┤etc) are designed to connect between
lines but the font's em-box is smaller than our line-height (14px vs
16.8px), creating visible gaps.

Solution: Render box-drawing characters as separate text elements with
a vertical scale transform of 1.2 (matching line-height) to stretch
them to fill the full cell height and connect properly.

This fixes disconnected vertical lines and corners in TUI applications.

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "textual-webterm"
-version = "0.3.13"
+version = "0.3.14"
 description = "Serve terminal sessions over the web"
 authors = ["Will McGugan <will@textualize.io>"]
 license = "MIT"