[PierreRaybaut]

Stacked on #374.

Applications window

• A full-height strip between the application list and the application page hides the list or shows it again. The window shrinks or grows by the list width, and the choice is remembered (applications_list_collapsed option).
• The methods, tools and datasets of an application form a standard QToolBox accordion: one section is open at a time, the first method by default. A colored dot on each method tells whether the current selection can be analyzed, and its tooltip gives the reason.

Welcome page

• Horizontal header (logo, slogan with the version, "Desktop edition of DataLab"), small uppercase section titles, compact rows with 24 px icons, framed walkthrough cards, a ▾ caret on the entries opening a menu (Create, Open file, Import text data), and a separator above the startup check box.

Tests

• welcome_unit_test.py and datalab/tests/features/plugins: 78 passed.
• New test for the list collapse (window and page widths, persistence); accordion assertions (items, default item, readiness dot and tooltip).

Docs: overview page and 1.4 release notes, with their French catalogs; datalab.po updated.

Assisted-by: Claude Opus 5.5