Task: Implement the AppImage Double-Click Handler feature

TASK-DESCRIPTION

Modify: sources/tools/appimage_integrate.cpp
Create: sources/tools/appimage_activator_ui.cpp
Modify: sources/CMakeLists.txt
Modify: scripts/program-install.sh
Modify: Makefile Add the handle, handler, and activator commands, and implement the requirements in prompts/features/10-appimage-handler.md. The activator is built only when the GTK4 development files are present, so the command-line tools still build without them. Registration records the previous default for each AppImage MIME type before it changes anything, so handler uninstall can restore it.

TASK-OUTPUT

Produce the complete new content of these files in this order, then the VERIFICATION: line: 1. sources/tools/appimage_integrate.cpp 2. sources/tools/appimage_activator_ui.cpp 3. sources/CMakeLists.txt 4. scripts/program-install.sh 5. Makefile

TASK-CONTEXT

The user authorised registering this tool as the real *.AppImage default, which is a live-state change and not part of make test. AppImageLauncher owned the AppImage MIME definitions; a user-space copy at $XDG_DATA_HOME/mime/packages/appimage.xml preserves them if the package is removed.

TASK-FILES

Operation	Path
modify	`sources/tools/appimage_integrate.cpp`
create	`sources/tools/appimage_activator_ui.cpp`
modify	`sources/CMakeLists.txt`
modify	`scripts/program-install.sh`
modify	`Makefile`

TASK-VERIFY

Run: make test from the repository root.
Expected: exit status 0, and tests/95-activator-ui.sh reports ok or skips when GTK4 is absent.

TASK-FEATURES

prompts/features/10-appimage-handler.md

TASK-ACCEPTANCE

APPIMAGE-HANDLER-R001
APPIMAGE-HANDLER-R003
APPIMAGE-HANDLER-R009
APPIMAGE-HANDLER-R010
APPIMAGE-HANDLER-R011
APPIMAGE-HANDLER-R013
APPIMAGE-HANDLER-R017
APPIMAGE-HANDLER-R035

OUTPUT: the five complete files in the order listed, ending with the VERIFICATION: line.