Task: Present the viewer as a three-tab panel

TASK-DESCRIPTION

Create: sources/tools/viewer_content.h
Create: sources/tools/viewer_content.cpp
Create: tests/viewer_content_test.cpp
Create: tests/82-viewer-content.sh
Modify: sources/tools/vcard_viewer_ui.cpp
Modify: sources/CMakeLists.txt
Modify: prompts/features/06-command-line-tools.md
Modify: prompts/tasks/00-tasks.md
Modify: README.md
Modify: TODO.md

vcard-viewer must present three tabs.

vCard keeps the contact page the viewer shows today.
Discovered runs vcard-inspect FILE and shows the exact command line and the combined output the tool printed, so the graphical view has a command-line equivalent on screen.
About shows the project name, the build-time version, and the GitHub repository link.

The non-GTK content is a small library so it can be tested without a display: the shell quoting of the command, the exact command and output capture, and the About identity. The viewer links that library and puts the three pages in a GtkNotebook. The vCard page is unchanged.

TASK-OUTPUT

Produce these complete files in this order, then the VERIFICATION: line: 1. sources/tools/viewer_content.h 2. sources/tools/viewer_content.cpp 3. tests/viewer_content_test.cpp 4. tests/82-viewer-content.sh 5. sources/tools/vcard_viewer_ui.cpp 6. sources/CMakeLists.txt 7. prompts/features/06-command-line-tools.md 8. prompts/tasks/00-tasks.md 9. README.md 10. TODO.md

TASK-CONTEXT

The viewer is built only where the GTK4 development files are present. The Discovered content runs the installed vcard-inspect beside the viewer, or one found on PATH, so it shows the command a person would type. The About repository link is https://github.com/pbannister/gnome-vcard-integration.

The vCard tab must keep the contact information the viewer already showed; do not move contact details into another tab or remove them.

TASK-FILES

Operation	Path
create	`sources/tools/viewer_content.h`
create	`sources/tools/viewer_content.cpp`
create	`tests/viewer_content_test.cpp`
create	`tests/82-viewer-content.sh`
modify	`sources/tools/vcard_viewer_ui.cpp`
modify	`sources/CMakeLists.txt`
modify	`prompts/features/06-command-line-tools.md`
modify	`prompts/tasks/00-tasks.md`
modify	`README.md`
modify	`TODO.md`

TASK-VERIFY

Run: make test from the repository root.
Expected: exit status 0, with 82-viewer-content and 95-viewer reporting ok.
Run: vcard-viewer dataflow.in/vcard/simple.vcf on a display.
Expected: a window with vCard, Discovered, and About tabs; the Discovered tab shows the inspector command and report; the About tab shows the version and repository link.

TASK-FEATURES

prompts/features/06-command-line-tools.md

TASK-ACCEPTANCE

COMMAND-LINE-TOOLS-R016
COMMAND-LINE-TOOLS-R017
COMMAND-LINE-TOOLS-R018
COMMAND-LINE-TOOLS-R019

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