Task: Clear the tab-selection focus in the viewer

TASK-DESCRIPTION

Modify: sources/tools/vcard_viewer_ui.cpp
Modify: prompts/features/06-command-line-tools.md
Modify: prompts/tasks/00-tasks.md
Modify: TODO.md

Selecting a tab makes GtkNotebook focus the first focusable child of the new page. In the vCard and About tabs that child is a selectable title label, so the title arrives selected. After the page switch settles, clear the window focus; the values remain selectable by hand, but opening a tab selects nothing. Clear the initial focus after the window is presented in the same way.

TASK-OUTPUT

Produce these complete files in this order, then the VERIFICATION: line: 1. sources/tools/vcard_viewer_ui.cpp 2. prompts/features/06-command-line-tools.md 3. prompts/tasks/00-tasks.md 4. TODO.md

TASK-CONTEXT

The focus is cleared with gtk_window_set_focus(window, NULL) from an idle callback connected to the notebook’s switch-page signal, and once after gtk_window_present.

Keep the labels selectable when a person clicks them; clear only the automatic focus and selection that a tab switch causes.

TASK-FILES

Operation	Path
modify	`sources/tools/vcard_viewer_ui.cpp`
modify	`prompts/features/06-command-line-tools.md`
modify	`prompts/tasks/00-tasks.md`
modify	`TODO.md`

TASK-VERIFY

Run: make test from the repository root.
Expected: exit status 0, with 95-viewer reporting ok.
Run: vcard-viewer dataflow.in/vcard/simple.vcf on a display and switch between the tabs.
Expected: no title is selected when a tab opens; clicking a label still selects it.

TASK-FEATURES

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

TASK-ACCEPTANCE

COMMAND-LINE-TOOLS-R020

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