Files · Items: one icon map, file icon, file row and file grid

The crate had two maps from a file's name to its icon, in chat and in the
file tree, and they disagreed. One map now lives in primitives, over the
Lucide file-* icons, and chat, agent, the file tree and the directory
listing use it. FileIcon draws it for a file or a folder. FileItem is a
file's row, a ListItem with its size and when it changed. FileList is the
directory listing. FileGrid lays tiles in the order given, with a cursor
seeded from the owner's pick: arrows by tile and by row, Home, End, Enter
and a double press to open. The count of what fits a width has one home,
layout's columns_for, and fit builds on it.

src/layout/mod.rs

@@ -19,6 +19,7 @@ pub use collapse::{Accordion, AccordionItem, Collapsible};
 pub use dock::{Dock, DockLayout, DockPanel, DockSide, Spot};
 pub use floating::FloatingPanel;
 pub use grid::{Masonry, SimpleGrid};
+pub(crate) use grid::{columns_for, fit};
 pub use page::{AppShell, MasterDetail, Page};
 pub use panes::{PaneGroup, PaneId, PaneLayout};
 pub(crate) use scroll::bring_into_view;