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/chat/mod.rs

@@ -28,7 +28,6 @@ pub use code::CodeBlock;
 pub use composer::{AttachmentButton, DragDropOverlay, InputHint, PromptInput, SendButton};
 pub use conversations::{Conversation, ConversationItem, ConversationList, new_chat_button};
 pub use list::{DateSeparator, MessageList, ScrollToBottomButton};
-pub(crate) use media::file_icon;
 pub use media::{FileMessage, ImageGrid, ImageMessage, LinkPreviewCard};
 pub use message::{
     ChatContainer, MessageAvatar, MessageBubble, MessageFooter, MessageHeader, Role,