PDFIMPORTIMAGEPATH and per-import layer prefixes

- PDFIMPORTIMAGEPATH names the image extraction folder: relative to the
  PDF's folder (default "PDF Images"), absolute as given, "." for none.
- Every import of the session gets the next prefix (PDF_, PDF2_, PDF3_ ...),
  also when the same file is imported again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/commands/mod.rs

@@ -622,6 +622,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "PDFIMPORTMODE",
         "PDFIMPORTFILTER",
         "PDFIMPORTLAYERS",
+        "PDFIMPORTIMAGEPATH",
         "XDWGFADECTL",
         "POINTCLOUDCLIPFRAME",
         "XCLIPFRAME",