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>

data/org.udpscope.app.desktop.in

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Name=udpscope
+GenericName=Oscilloscope
+Comment=An oscilloscope for UDPStreamer sources
+Exec=@UDPSCOPE_EXEC@
+Icon=org.udpscope.app
+Terminal=false
+Categories=Development;Science;Electronics;
+Keywords=oscilloscope;UDP;signal;waveform;trigger;
+StartupNotify=true