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/20-upload.css

@@ -48,6 +48,10 @@
   width: 100%;
 }
 
+.upload-options .form-footer .upload-new-button {
+  margin-top: -0.25rem;
+}
+
 .hero-copy {
   text-align: center;
 }
@@ -371,6 +375,62 @@ button {
   background: var(--surface-1-hover);
 }
 
+.upload-file-waiting {
+  border-color: color-mix(in srgb, var(--primary) 42%, var(--border));
+  background: color-mix(in srgb, var(--primary) 8%, var(--background));
+}
+
+.upload-file-complete .file-progress span {
+  background: var(--success);
+}
+
+.upload-file-state {
+  grid-column: 1 / -1;
+  color: var(--muted-foreground);
+  font-size: 0.72rem;
+  text-align: right;
+}
+
+.upload-recovery-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 80;
+  display: grid;
+  place-items: center;
+  padding: 1rem;
+  background: color-mix(in srgb, var(--background) 72%, transparent);
+  backdrop-filter: blur(10px);
+}
+
+.upload-recovery-modal {
+  width: min(34rem, 100%);
+  box-shadow: var(--shadow-lg);
+}
+
+.upload-recovery-modal h2 {
+  margin: 0 0 0.65rem;
+  font-size: 1.35rem;
+}
+
+.upload-recovery-modal p {
+  margin: 0;
+  color: var(--muted-foreground);
+  line-height: 1.55;
+}
+
+.upload-recovery-actions {
+  display: grid;
+  grid-template-columns: repeat(2, minmax(0, 1fr));
+  gap: 0.7rem;
+  margin-top: 1.25rem;
+}
+
+@media (max-width: 640px) {
+  .upload-recovery-actions {
+    grid-template-columns: 1fr;
+  }
+}
+
 .result-item small,
 .download-item small,
 .result-item code,