Fix character alignment with textLength attribute

Use SVG textLength and lengthAdjust='spacingAndGlyphs' to enforce
exact character spacing, preventing gaps between box-drawing and
other characters that may render at slightly different widths.

Also include whitespace spans in output for proper alignment.

src/textual_webterm/svg_exporter.py

@@ -191,8 +191,8 @@ def render_terminal_svg(
                     f'fill="{span["bg"]}"/>'
                 )
 
-            if not text or (text.isspace() and not span["has_bg"]):
-                # Skip empty spans without background, but advance position
+            if not text:
+                # Skip truly empty spans (wide char placeholders already counted)
                 x += columns * char_width
                 continue
 
@@ -208,14 +208,19 @@ def render_terminal_svg(
         for span in spans:
             text = span["text"]
             columns = span["columns"]
-            if not text or (text.isspace() and not span["has_bg"]):
-                # Skip empty spans without background, but advance position
+            if not text:
+                # Skip truly empty spans
                 x += columns * char_width
                 continue
 
             # Build tspan attributes
             attrs = [f'x="{x:.1f}"']
 
+            # Use textLength to enforce exact character spacing for alignment
+            span_width = columns * char_width
+            attrs.append(f'textLength="{span_width:.1f}"')
+            attrs.append('lengthAdjust="spacingAndGlyphs"')
+
             # Foreground color
             if span["fg"] != foreground:
                 attrs.append(f'fill="{span["fg"]}"')