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.

examples/gallery/pages/mod.rs

@@ -8,6 +8,7 @@ mod debug;
 mod documents;
 mod editor;
 mod feedback;
+mod files;
 mod finance;
 mod forms;
 mod generative;
@@ -67,6 +68,7 @@ pub const ALL: &[Page] = &[
     agent::PAGE,
     generative::PAGE,
     media::PAGE,
+    files::PAGE,
     theme::PAGE,
 ];