Fix #205: strip whitespace-only text nodes between codeline blocks

paste-lines.js rebuilt the codebody with innerHTML but relied on the
server-side '\n' delimiters disappearing between the per-line blocks.
Under white-space: pre any whitespace-only text node that survives
(cached HTML, template drift) renders as a phantom row and drifts the
gutter numbers off their lines. splitLines() now strips whitespace-only
text nodes after rebuilding, keeping renumber() geometry intact.

internal/lang/highlight.go

@@ -146,5 +146,9 @@ func HighlightCode(content, langID string) string {
 	for i, line := range lines {
 		out[i] = highlightLine(line, h, langID)
 	}
+	// Note: lines are joined with "\n" on purpose — paste-lines.js splits on
+	// the newline to build its per-line .codeline blocks, then joins those
+	// with "" and strips whitespace-only text nodes (#205), so no newline
+	// text node ever reaches the rendered DOM.
 	return strings.Join(out, "\n")
 }