feat: a desktop entry, installed by an opt-in CMake target

The entry's Exec is the built binary's absolute path, so it always launches what
was last built -- and moving the build directory breaks it, which is the price
of having no install step. An install() rule would make a second copy that can
diverge; a documented cp would need the binary on PATH, which it is not.

The target writes the entry and both icons under XDG_DATA_HOME, refreshes the
desktop and icon caches when the tools are there, and runs
desktop-file-validate as a hard failure -- the shell ignores a malformed entry
in silence, so the failure belongs at the moment the entry is asked for. It
prints the three paths it wrote, which is the uninstall instructions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CMakeLists.txt

@@ -126,3 +126,30 @@ add_dependencies(udpscope udpscope_build_info)
 if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
   target_compile_options(udpscope PRIVATE -Wall -Wextra -Wpedantic)
 endif()
+
+# --- the desktop entry: opt-in, never part of `cmake --build` ------------------
+# `Exec` is the built binary's absolute path in the build tree, so the entry always launches
+# exactly the binary that was last built -- and moving or deleting the build directory breaks
+# it. That is the price of having no install step, and it is the right one here: an install()
+# rule would create a second copy of the app that can diverge from the build tree, and a
+# documented `cp` would need the binary on PATH, which it is not.
+#
+# configure_file() cannot evaluate a generator expression and file(GENERATE) cannot substitute
+# an @VAR@, so the two run in sequence: the first writes the genex out literally, the second
+# evaluates it. This is the documented idiom.
+set(UDPSCOPE_EXEC "$<TARGET_FILE:udpscope>")
+configure_file(${CMAKE_CURRENT_SOURCE_DIR}/data/org.udpscope.app.desktop.in
+               ${CMAKE_CURRENT_BINARY_DIR}/desktop-entry.in @ONLY)
+file(GENERATE OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/org.udpscope.app.desktop
+     INPUT ${CMAKE_CURRENT_BINARY_DIR}/desktop-entry.in)
+
+# No ALL: this runs only when it is named, so `cmake --build build` is unaffected.
+add_custom_target(desktop-entry
+  COMMAND ${CMAKE_COMMAND}
+          -DENTRY=${CMAKE_CURRENT_BINARY_DIR}/org.udpscope.app.desktop
+          -DICON=${UDPSCOPE_ICON_DIR}/scalable/apps/org.udpscope.app.svg
+          -DSYMBOLIC=${UDPSCOPE_ICON_DIR}/scalable/apps/org.udpscope.app-symbolic.svg
+          -P ${CMAKE_CURRENT_SOURCE_DIR}/cmake/install_desktop_entry.cmake
+  COMMENT "Installing the udpscope desktop entry into XDG_DATA_HOME"
+  VERBATIM)
+add_dependencies(desktop-entry udpscope)