feat(upload): add resumable chunk configuration and file validation

- Add `WARPBOX_RESUMABLE_CHUNK_MODE` and `WARPBOX_RESUMABLE_CHUNK_PATH` environment variables to configure temporary chunk storage.
- Implement strict file validation for resuming uploads to ensure selected files match the pending session's metadata.
- Add `PLANS.md` to document development stages, roadmap, and API specifications (including batching and resumable flows).

backend/static/css/30-download.css

@@ -46,6 +46,15 @@
   text-decoration: none;
 }
 
+.upload-processing-alert {
+  margin: 1rem 0;
+  padding: .85rem 1rem;
+  border: 1px solid color-mix(in srgb, var(--primary) 45%, transparent);
+  border-radius: var(--radius);
+  background: color-mix(in srgb, var(--primary) 12%, transparent);
+  color: var(--foreground);
+}
+
 .thumb-link {
   flex: 0 0 4.75rem;
   width: 4.75rem;
@@ -200,6 +209,15 @@
   padding: 0;
 }
 
+.file-card.is-processing {
+  opacity: .62;
+  filter: grayscale(.25);
+}
+
+.file-card.is-processing .file-open {
+  cursor: wait;
+}
+
 .file-reaction-dock {
   position: static;
   z-index: 2;