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>

cmake/install_desktop_entry.cmake

@@ -0,0 +1,61 @@
+# SPDX-License-Identifier: MIT
+#
+# The `desktop-entry` target's body. Copies the generated entry and the two app icons into
+# $XDG_DATA_HOME, validates the entry, refreshes the caches that make the shell notice, and
+# prints every path it wrote.
+#
+# This runs only when the target is named. The project deliberately has no install step -- that
+# is the stated reason its icons are a GResource -- and a target that does nothing until asked
+# preserves that while still producing a working entry.
+#
+# Expects -DENTRY=, -DICON= and -DSYMBOLIC= on the command line.
+
+if(DEFINED ENV{XDG_DATA_HOME} AND NOT "$ENV{XDG_DATA_HOME}" STREQUAL "")
+  set(data_home "$ENV{XDG_DATA_HOME}")
+else()
+  set(data_home "$ENV{HOME}/.local/share")
+endif()
+
+set(apps_dir "${data_home}/applications")
+set(icon_dir "${data_home}/icons/hicolor/scalable/apps")
+set(sym_dir "${data_home}/icons/hicolor/symbolic/apps")
+
+# The icons are copied from the source tree rather than extracted from the GResource: the same
+# two files serve both the in-binary resource and the installed theme, so there is one copy to
+# edit. `file(COPY)` creates the directories.
+file(COPY "${ENTRY}" DESTINATION "${apps_dir}")
+file(COPY "${ICON}" DESTINATION "${icon_dir}")
+file(COPY "${SYMBOLIC}" DESTINATION "${sym_dir}")
+
+get_filename_component(entry_name "${ENTRY}" NAME)
+get_filename_component(icon_name "${ICON}" NAME)
+get_filename_component(sym_name "${SYMBOLIC}" NAME)
+
+# Validated here rather than left to the shell, which ignores a malformed entry in silence: the
+# failure belongs at the moment the entry is asked for.
+find_program(DESKTOP_FILE_VALIDATE desktop-file-validate)
+if(DESKTOP_FILE_VALIDATE)
+  execute_process(COMMAND "${DESKTOP_FILE_VALIDATE}" "${apps_dir}/${entry_name}"
+                  RESULT_VARIABLE validate_result)
+  if(NOT validate_result EQUAL 0)
+    message(FATAL_ERROR "desktop-file-validate rejected ${apps_dir}/${entry_name}")
+  endif()
+  message(STATUS "desktop-file-validate: ok")
+else()
+  message(STATUS "desktop-file-validate: not on PATH, entry not validated")
+endif()
+
+find_program(UPDATE_DESKTOP_DATABASE update-desktop-database)
+if(UPDATE_DESKTOP_DATABASE)
+  execute_process(COMMAND "${UPDATE_DESKTOP_DATABASE}" "${apps_dir}")
+endif()
+
+find_program(GTK4_UPDATE_ICON_CACHE gtk4-update-icon-cache)
+if(GTK4_UPDATE_ICON_CACHE)
+  execute_process(COMMAND "${GTK4_UPDATE_ICON_CACHE}" -f -t "${data_home}/icons/hicolor")
+endif()
+
+# Printed because there is no uninstall target: undoing this is these three paths.
+message(STATUS "wrote ${apps_dir}/${entry_name}")
+message(STATUS "wrote ${icon_dir}/${icon_name}")
+message(STATUS "wrote ${sym_dir}/${sym_name}")