PDF links: extract pdf_links.{cpp,h} shared helper

Move the PDF cross-reference hyperlink logic out of ProjectPrintWindow
into a standalone translation unit so it can be reused (the CLI PDF
export will call it next):

- injectCrossRefLinks(): emits the URI link annotations for a diagram's
  cross-references and folio reports. The scene-to-page mapping is passed
  in as a PageGeometry (transform + devToPdf + source-rect lookup) so each
  caller supplies its own correct geometry, rather than the helper assuming
  a QPrinter.
- convertUriToGoTo(): the PDF post-processor, moved verbatim.

ProjectPrintWindow stays a pure caller: it builds its PageGeometry from the
printer page layout exactly as before and calls the helper. No behavioural
change to GUI PDF export; no class-structure changes.

Per review guidance on #483.

cmake/qet_compilation_vars.cmake

@@ -109,6 +109,8 @@ set(QET_RES_FILES
 set(QET_SRC_FILES
   ${QET_DIR}/sources/cli_export.cpp
   ${QET_DIR}/sources/cli_export.h
+  ${QET_DIR}/sources/pdf_links.cpp
+  ${QET_DIR}/sources/pdf_links.h
   ${QET_DIR}/sources/borderproperties.cpp
   ${QET_DIR}/sources/borderproperties.h
   ${QET_DIR}/sources/bordertitleblock.cpp