feat: accept any file drag into the chat input

[wuxiangru915]

Summary

The chat input currently only accepts image drags (useDragDrop gates on image/*, and ChatInput.addImages filters to images). Dropping any other file silently does nothing. This PR makes the drop zone accept any file from the OS file manager.

Behavior

• Images — unchanged: attached as base64 multimodal content and sent with the message.
• Any other file / folder — its real filesystem path is inserted into the input at the caret, so the agent can read it with its own tools. Browsers do not expose the real path on File objects, but OS-originated drags put file:// URIs on the dataTransfer's text/uri-list; since pi-web is a local app, that path is directly usable. Falls back to the bare file name when no path is available (e.g. drag from another browser tab).
• The full-window drop overlay now shows a document icon instead of the photo icon when the drag contains no images.

Why path insertion (not file upload)

pi's agent core prompt() only supports images attachments, so arbitrary file content cannot be attached end-to-end. Inserting the path lets the agent fetch the file itself and works for every file type, any size.

Implementation

• lib/dropped-files.ts (new): parses text/uri-list file:// URIs (POSIX + Windows drive paths, URL decoding, comment/blank lines), a single-file text/plain fallback, and name fallback. Unit-tested in lib/dropped-files.test.mjs.
• hooks/useDragDrop.ts: accept any kind === "file" drag, expose dragHasImages, pass the dataTransfer to the drop callback (needed to read text/uri-list, only readable at drop time).
• components/ChatInput.tsx: new addFiles(files, dataTransfer) imperative method (images → existing attach flow; others → path insertion). The insertText logic is factored into a reusable insertTextAtCursor.
• components/ChatWindow.tsx: routes drops to addFiles, swaps the overlay icon based on dragHasImages.
• hooks/useAgentSession.ts: ChatInputHandle interface updated in sync.

Verification

• tsc --noEmit clean
• eslint clean on changed files
• New unit tests pass (8/8); existing jiti-based tests pass, including the SSR ChatInput.test.mjs