feat(upload): add transfer rate tracking and 6-hour expiry option

- Implement real-time transfer rate tracking and display upload speed (e.g., Mb/s) in the progress status.
- Add a 6-hour (360 minutes) option to the upload expiry selection ladder.
- Fix an issue where the "new upload" button remained visible by explicitly toggling its display style and adding a CSS fallback for the `hidden` attribute.

backend/static/js/40-upload.js

@@ -117,9 +117,7 @@
         uploadQueue.hidden = true;
         uploadQueue.replaceChildren();
       }
-      if (newUpload) {
-        newUpload.hidden = true;
-      }
+      updateNewUploadVisibility();
       if (fileSummary) {
         fileSummary.textContent = "Upload complete.";
       }
@@ -189,9 +187,16 @@
       uploadQueue.hidden = true;
       uploadQueue.replaceChildren();
     }
-    if (newUpload) {
-      newUpload.hidden = !(resumeMode && recoveredDraft);
+    updateNewUploadVisibility();
+  }
+
+  function updateNewUploadVisibility() {
+    if (!newUpload) {
+      return;
     }
+    const visible = Boolean(resumeMode && recoveredDraft);
+    newUpload.hidden = !visible;
+    newUpload.style.display = visible ? "" : "none";
   }
 
   function setLoading(isLoading, submit) {
@@ -216,6 +221,41 @@
     }
   }
 
+  function updateUploadProgress(percent, bytesPerSecond) {
+    const clamped = Math.max(0, Math.min(100, Math.round(percent || 0)));
+    const rate = formatTransferRate(bytesPerSecond);
+    updateStatus(rate ? `${clamped}% · ${rate}` : `${clamped}%`);
+  }
+
+  function createTransferRateTracker(initialBytes) {
+    const startedAt = performance.now();
+    const baseline = Math.max(0, initialBytes || 0);
+    let lastRate = 0;
+    return function track(currentBytes) {
+      const elapsedSeconds = (performance.now() - startedAt) / 1000;
+      const transferred = Math.max(0, (currentBytes || 0) - baseline);
+      if (elapsedSeconds < 0.25 || transferred <= 0) {
+        return lastRate;
+      }
+      lastRate = transferred / elapsedSeconds;
+      return lastRate;
+    };
+  }
+
+  function formatTransferRate(bytesPerSecond) {
+    if (!Number.isFinite(bytesPerSecond) || bytesPerSecond <= 0) {
+      return "";
+    }
+    const units = ["b/s", "Kb/s", "Mb/s", "Gb/s"];
+    let value = bytesPerSecond * 8;
+    let unit = 0;
+    while (value >= 1000 && unit < units.length - 1) {
+      value /= 1000;
+      unit += 1;
+    }
+    return `${value >= 10 || unit === 0 ? value.toFixed(0) : value.toFixed(1)} ${units[unit]}`;
+  }
+
   function renderResult(payload) {
     if (!result || !resultList || !resultMeta || !openBox) {
       return;
@@ -248,16 +288,18 @@
   function uploadWithProgress(url, formData, files) {
     return new Promise((resolve, reject) => {
       const request = new XMLHttpRequest();
+      const rateTracker = createTransferRateTracker(0);
       request.open("POST", url);
       request.setRequestHeader("Accept", "application/json");
 
       request.upload.addEventListener("progress", (event) => {
+        const rate = rateTracker(event.loaded || 0);
         if (!event.lengthComputable) {
-          updateStatus("Uploading...");
+          updateStatus(rate > 0 ? `Uploading · ${formatTransferRate(rate)}` : "Uploading...");
           return;
         }
         const percent = Math.round((event.loaded / event.total) * 100);
-        updateStatus(`${percent}%`);
+        updateUploadProgress(percent, rate);
         setTotalProgress(percent);
         setFileProgress(files, percent);
       });
@@ -348,7 +390,9 @@
       completedByFile[index] = uploadedBytesForSessionFile(sessionFile, session.chunkSize);
       setSingleFileProgress(index, files[index], percentForBytes(completedByFile[index], files[index].size));
     });
-    setTotalProgress(percentForBytes(completedByFile.reduce((sum, bytes) => sum + bytes, 0), totalBytes));
+    const initiallyUploadedBytes = completedByFile.reduce((sum, bytes) => sum + bytes, 0);
+    const rateTracker = createTransferRateTracker(initiallyUploadedBytes);
+    setTotalProgress(percentForBytes(initiallyUploadedBytes, totalBytes));
 
     for (let fileIndex = 0; fileIndex < files.length; fileIndex++) {
       const file = files[fileIndex];
@@ -362,9 +406,11 @@
         const end = Math.min(file.size, start + session.chunkSize);
         await uploadChunkWithRetry(session, sessionFile, chunkIndex, file.slice(start, end), (loaded) => {
           const currentTotal = completedByFile.reduce((sum, bytes) => sum + bytes, 0) + loaded;
-          setTotalProgress(percentForBytes(currentTotal, totalBytes));
+          const percent = percentForBytes(currentTotal, totalBytes);
+          const rate = rateTracker(currentTotal);
+          setTotalProgress(percent);
           setSingleFileProgress(fileIndex, file, percentForBytes(completedByFile[fileIndex] + loaded, file.size));
-          updateStatus(`${percentForBytes(currentTotal, totalBytes)}%`);
+          updateUploadProgress(percent, rate);
         });
         completedByFile[fileIndex] += end - start;
         uploaded.add(chunkIndex);
@@ -749,6 +795,7 @@
     selectedFiles = [];
     renderResumeQueue(recoveredDraft.session, selectedFiles);
     updateSelectedState();
+    updateNewUploadVisibility();
     updateStatus("Drop or reselect missing files to continue. Extra files will be added to this upload.");
   }