feat(frontend): implement file upload with drag-and-drop and per-file progress
- client.ts: add uploadWithProgress() using XHR for upload progress events
- FileUpload.svelte: drag-drop zone wrapper, multi-file queue with individual
progress bars, success/error status, MIME rejection message, dismiss panel
- Header.svelte: optional onUpload prop renders upload icon button
- files/+page.svelte: wire upload button, prepend uploaded files to grid
- vite-mock-plugin.ts: handle POST /files, unshift new file into mock array
- Fix crypto.randomUUID() crash on non-secure HTTP context (use Date.now + Math.random)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Masahiko AMANO
b9cace2997