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/00-utils.js

@@ -5,6 +5,17 @@
     window.open(url, "_blank", "noopener,noreferrer");
   };
 
+  window.Warpbox.absoluteURL = function absoluteURL(url) {
+    if (!url) {
+      return "";
+    }
+    try {
+      return new URL(url, window.location.origin).href;
+    } catch (_) {
+      return url;
+    }
+  };
+
   window.Warpbox.writeClipboard = async function writeClipboard(text) {
     if (navigator.clipboard && window.isSecureContext) {
       await navigator.clipboard.writeText(text);
@@ -26,6 +37,9 @@
     if (!text || !button) {
       return;
     }
+    if (typeof text === "string" && (text.startsWith("/") || /^https?:\/\//i.test(text))) {
+      text = window.Warpbox.absoluteURL(text);
+    }
     await window.Warpbox.writeClipboard(text);
     const previous = button.textContent;
     button.textContent = copiedLabel;