feat(uploads): add native resumable upload support

Implement a native chunked resumable upload API and frontend integration
to support reliable large file uploads.

Changes include:
- Added a 3-step resumable upload API flow (create session, upload chunks, complete session).
- Introduced configuration options for chunk size, retention hours, and toggling the feature.
- Updated the frontend to utilize resumable uploads with progress tracking.
- Configured temporary chunk storage under `data/tmp/uploads` with automatic cleanup.
- Documented the API flow and configuration in the README.

backend/static/css/20-upload.css

@@ -335,10 +335,13 @@ button {
 .file-progress-side {
   width: min(10rem, 32vw);
   display: grid;
+  grid-template-columns: minmax(0, 1fr) auto;
   gap: 0.35rem;
+  align-items: center;
 }
 
 .file-progress-percent {
+  grid-column: 1 / -1;
   color: var(--muted-foreground);
   font-size: 0.75rem;
   text-align: right;
@@ -349,6 +352,25 @@ button {
   margin-top: 0;
 }
 
+.upload-file-remove {
+  width: 1.65rem;
+  height: 1.65rem;
+  min-height: 1.65rem;
+  padding: 0;
+  border-color: var(--border);
+  border-radius: 999px;
+  color: var(--muted-foreground);
+  background: var(--surface-1);
+  font-size: 1rem;
+  line-height: 1;
+}
+
+.upload-file-remove:hover {
+  color: var(--foreground);
+  border-color: var(--primary);
+  background: var(--surface-1-hover);
+}
+
 .result-item small,
 .download-item small,
 .result-item code,