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/js/10-file-browser.js

@@ -124,7 +124,7 @@
   }
 
   async function copyPreviewLink(button) {
-    await window.Warpbox.writeClipboard(button.href);
+    await window.Warpbox.writeClipboard(window.Warpbox.absoluteURL(button.href));
     const label = button.querySelector("[data-preview-label]");
     if (!label) {
       return;
@@ -164,11 +164,11 @@
       return true;
     }
     if (action === "copy-preview") {
-      await window.Warpbox.writeClipboard(file.previewURL);
+      await window.Warpbox.writeClipboard(window.Warpbox.absoluteURL(file.previewURL));
       return true;
     }
     if (action === "copy-download") {
-      await window.Warpbox.writeClipboard(file.downloadURL);
+      await window.Warpbox.writeClipboard(window.Warpbox.absoluteURL(file.downloadURL));
       return true;
     }
     if (action === "download") {