feat(upload): support presigned multipart manifest for S3 uploads

- Updated `js/s3-quick.js` to support new manifest structure: `{ bucket, key, uploadId, parts: [{partNumber, url}] }`.
- Added logic to sort parts, upload chunks, collect ETags, and call `completeUrl` if present.
- Updated `js/ui/components/UploadModal.js` to handle manifest flow, validate new structure, and alert user if completion is manual (missing `completeUrl`).
- Updated `components/upload-modal.html` with new placeholder example and documentation note.
- Verified logic with reproduction script and UI with Playwright.

components/upload-modal.html

@@ -319,11 +319,14 @@
 
                                 <div id="quickS3ManifestContainer" class="hidden space-y-2">
                                     <label for="quickS3ManifestJson" class="text-xs font-medium text-text">Manifest JSON</label>
+                                    <p class="text-4xs text-muted leading-tight">
+                                        The manifest must be produced externally. This app does not create or sign presigned URLs.
+                                    </p>
                                     <textarea
                                         id="quickS3ManifestJson"
                                         rows="6"
                                         class="textarea text-xs font-mono"
-                                        placeholder='{"uploadId": "...", "partUrls": ["..."]}'
+                                        placeholder='{"bucket": "...", "key": "...", "uploadId": "...", "parts": [{"partNumber": 1, "url": "..."}]}'
                                     ></textarea>
                                 </div>