feat(proxy): add offset param to inset or outset traced shapes

Trace results can now be inset (negative) or outset (positive) by a
pixel amount. Offset the outline and holes in opposite directions with
clipper-lib's miter joins, then recombine with a boolean difference so
holes grow on inset and shrink on outset. Collapsed shapes return an
empty outline; a split outline keeps the largest ring. Document the
parameter and the new dependency.

README.md

@@ -38,6 +38,7 @@ To run the frontend alongside the proxy, open a second terminal and run
 | GET    | `/search?q=&page=`| Search the Workshop (scrapes browse page)    |
 | GET    | `/items/:id`      | Full parsed `TTSMod` (BSON save)             |
 | GET    | `/items/:id/file` | Raw save bytes, filename from header         |
+| GET    | `/trace?url=&mode=&format=&offset=` | Trace an image into a vector shape (BSON); `offset` insets/outsets in pixels |
 
 ## Commands