Menus are `position: fixed`, so only the window can clip them, but they
were placed at the raw cursor position with a naive flip: near the bottom
edge the menu opened below the window (nothing to scroll it into view),
and a tall menu that flipped up could land above the top edge. The
sidebar folder menu and the view-options menu did no viewport math at all,
so right-clicking a folder at the bottom of the tree showed no options.
Add a shared positionFixedMenu helper that picks the side with room,
clamps on both axes, and caps the height (scrolling internally) when the
menu is taller than the window. BaseContextMenu uses it and exposes a
prepareMenu hook so per-card entries are toggled before the menu is
measured; the Recipe/Lora/Checkpoint/Embedding/Other/Workflow menus used
to mutate the menu after positioning and so measured the previous
content. Submenus get the same treatment via positionSubmenu.
Fixes#1167
