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/primitives/icon.rs

@@ -129,9 +129,19 @@ icons! {
     ExternalLink => "external-link",
     EyeOff => "eye-off",
     Eye => "eye",
+    FileArchive => "file-archive",
+    FileAudio => "file-audio",
+    FileCode => "file-code",
+    FileCog => "file-cog",
+    FileImage => "file-image",
+    FileJson => "file-json",
     FilePlus => "file-plus",
+    FileSpreadsheet => "file-spreadsheet",
+    FileTerminal => "file-terminal",
     FileText => "file-text",
     File => "file",
+    FileType => "file-type",
+    FileVideoCamera => "file-video-camera",
     Filter => "filter",
     Fingerprint => "fingerprint",
     Flag => "flag",