feat: paged help window with the four Getting started pages

The pages are Pango markup in a GResource, bundled for the same reason the icons
are: udpscope runs straight out of the build tree with no install step, so a
runtime search path's failure mode would be a silently empty help window.
--c-name=udpscope_help keeps the two generated bundles from colliding.

Every page is validated with pango_parse_markup() at load and falls back to a
one-line stand-in rather than rendering raw markup or nothing. show_help() reuses
help_window.cpp's existing close-request handler, and closes via
gtk_window_close() so that handler runs and the pointer is nulled.

UDPSCOPE_PROBE_HELP_MS loads and validates every page, opens the help and About
windows, closes both and reports; parsed must equal pages.

CMakeLists.txt

@@ -51,6 +51,23 @@ add_custom_command(
   COMMENT "Compiling udpscope icon resources"
   VERBATIM)
 
+# --- help pages: Pango markup in a GResource ----------------------------------
+# A second bundle beside the icons'. --c-name is given explicitly so the two generated
+# sources cannot collide on symbol names.
+set(UDPSCOPE_HELP_DIR ${CMAKE_CURRENT_SOURCE_DIR}/src/help)
+file(GLOB UDPSCOPE_HELP_FILES CONFIGURE_DEPENDS ${UDPSCOPE_HELP_DIR}/*.txt)
+add_custom_command(
+  OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/udpscope_help_resources.c
+  COMMAND ${GLIB_COMPILE_RESOURCES}
+          --target=${CMAKE_CURRENT_BINARY_DIR}/udpscope_help_resources.c
+          --sourcedir=${UDPSCOPE_HELP_DIR}
+          --c-name=udpscope_help
+          --generate-source
+          ${UDPSCOPE_HELP_DIR}/udpscope-help.gresource.xml
+  DEPENDS ${UDPSCOPE_HELP_DIR}/udpscope-help.gresource.xml ${UDPSCOPE_HELP_FILES}
+  COMMENT "Compiling udpscope help resources"
+  VERBATIM)
+
 # --- build info: version, commit and timestamp compiled into the binary -------
 # Written at configure time so add_executable can name the file, and again on every
 # build so the commit and the timestamp belong to the binary that is running rather
@@ -72,6 +89,7 @@ set_source_files_properties(${UDPSCOPE_BUILD_INFO} PROPERTIES GENERATED TRUE)
 # --- the application ----------------------------------------------------------
 add_executable(udpscope
   ${CMAKE_CURRENT_BINARY_DIR}/udpscope_resources.c
+  ${CMAKE_CURRENT_BINARY_DIR}/udpscope_help_resources.c
   ${UDPSCOPE_BUILD_INFO}
   src/main.cpp
   src/config.cpp