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/api/ratelimit_test.go

@@ -166,8 +166,11 @@ func TestHighlightCode(t *testing.T) {
 	if plain != "<b>x</b>" {
 		t.Fatalf("plain escaping wrong: %q", plain)
 	}
-	// line count preserved (gutter alignment)
-	if got := len(splitLines(lang.HighlightCode("a\nb\nc", "go"))); got != 3 {
+	// #205: newline join preserved as the delimiter paste-lines.js splits on;
+	// per-line segments survive and the client joins with '' so no newline
+	// text node reaches the rendered DOM.
+	hl := lang.HighlightCode("a\nb\nc", "go")
+	if got := len(splitLines(hl)); got != 3 {
 		t.Fatalf("want 3 lines, got %d", got)
 	}
 }