feat(snap): mark the parallel reference line + toggle it off on re-hover

Two refinements to Parallel snap. The acquired reference now keeps a
point on the line and draws a ∥ marker there, so the user sees which line
the parallel is measured from. And dwelling on that same reference line
again removes it (toggle), matching how OTRACK tracking points clear;
dwelling on a different line switches the reference. Same-line detection
uses parallel direction plus perpendicular screen distance, so it holds
across zoom.

Refs #277

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update/viewport.rs

@@ -951,7 +951,7 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                             eye,
                             bounds,
                         ) {
-                            if let (Some(base), Some(dir)) =
+                            if let (Some(base), Some((dir, _))) =
                                 (self.last_point, self.snapper.parallel_ref)
                             {
                                 self.otrack_active = Some((base, dir));