feat(clipboard): paste menu with paste-to-original and paste-as-block

Turn the ribbon Clipboard "Paste" button into a dropdown:
- Paste — existing pick-a-point paste.
- Paste to Original Coordinates — drops the clipboard at its source
  coordinates with no prompt.
- Paste as Block — wraps the clipboard contents in a new block definition
  and starts an interactive insert (prompts for the drop point, with the
  geometry rubber-banding under the cursor).

Also fixes two bugs the new commands exposed:
- entities_centroid summed the NaN separator points wire models carry,
  so the paste base point came out NaN; non-finite points are now
  skipped. A NaN base previously froze block tessellation outright.
- PASTEORIG added clipboard entities with their original handles
  (duplicates on same-document paste); it now goes through
  add_entity_clone and merges referenced records like the other pastes.

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

src/app/helpers.rs

@@ -186,8 +186,14 @@ pub(super) fn entities_centroid(wires: &[WireModel]) -> glam::Vec3 {
     let mut count = 0usize;
     for w in wires {
         for p in &w.points {
-            sum += glam::Vec3::from(*p);
-            count += 1;
+            let v = glam::Vec3::from(*p);
+            // Wire models carry NaN points as separators between disjoint
+            // segments; summing them poisons the whole centroid into NaN,
+            // which then makes every paste base point NaN. (#129)
+            if v.is_finite() {
+                sum += v;
+                count += 1;
+            }
         }
     }
     if count > 0 {