Fix file attachment API format and duplicate attachments

- Add required 'url' field to file parts in API request
- Use url, mime, filename instead of path field
- Prevent duplicate attachments by checking if path already exists
- Show all files when picker opens with empty query (use space as query)
- Filter git files only when search query provided
- Enter key now works correctly when file list has items

src/stores/sessions.ts

@@ -652,8 +652,9 @@ async function sendMessage(
       if (source.type === "file") {
         parts.push({
           type: "file" as const,
-          path: source.path,
+          url: att.url,
           mime: source.mime,
+          filename: att.filename,
         })
       } else if (source.type === "text") {
         parts.push({