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/templates/pages/home.html

@@ -77,6 +77,7 @@
         <div class="form-footer">
           <p id="file-summary">Choose one or more files to begin.</p>
           <button class="button button-primary" type="submit">Upload files</button>
+          <button class="button button-danger upload-new-button" type="button" id="new-upload" hidden>New upload</button>
         </div>
       </div>
     </div>