Move list-window loading bar to a slim always-visible footer strip

EntityListMdiWindow::loadingBar() (EntityListMdiWindow.cpp) currently creates a full-width QProgressBar in indeterminate (marquee) mode, placed between the toolbar and the table, that is hidden by default and only show()=n while a fetch is in flight. Replace it with a slim, fixed-height progress strip pinned to the bottom of the window (below the =PaginationWidget, or merged into that same footer row) that is always visible — present but visually quiet when idle, and animating during a load — rather than appearing and disappearing and shifting the layout each time.

A bar that pops in and out of the layout on every reload is a minor but repeated visual disruption (nearby widgets shift down when it appears), and gives the loading state less consistent screen real estate than it deserves given how frequently list windows reload. A slim, always-present footer strip is a more standard "busy" affordance (cf. VS Code's / most browsers' bottom status bar) and avoids the layout shift entirely.

• projects/ores.qt/api/include/ores.qt/EntityListMdiWindow.hpp and projects/ores.qt/api/src/EntityListMdiWindow.cpp — current loadingBar() implementation (show/hide, indeterminate mode, fixed 20px height).
• projects/ores.qt/api/include/ores.qt/PaginationWidget.hpp — the existing bottom-row widget this would sit alongside or merge into.

• UI design principles — the design rulebook this change should be checked against.
• Qt entity patterns — the list-window pattern EntityListMdiWindow is the base class for; changing its loading affordance affects every generated list window.