A Mirror window for sketch entities, holding both picks until OK (#734)

Sketch Mirror now opens a tool window with Entities and Mirror line inputs, so both picks stay visible and editable until OK, replacing the old click-then-right-click flow.

crates/ui-egui/src/locales/de.json

@@ -196,6 +196,7 @@
   "ui.measure": "Messen",
   "ui.midpoint": "Mittelpunkt",
   "ui.mirror": "Spiegeln",
+  "ui.mirror_line": "Spiegelachse",
   "ui.modify": "Ändern",
   "ui.move": "Verschieben",
   "ui.multiple": "Mehrfach",