feat: add file upload progress toast with per-file tracking (#93)

Replaced the hidden dropzone-only progress bar with a floating
upload toast that appears at the bottom-right corner whenever
files are being uploaded (button or drag-and-drop).

Features:
- Per-file progress bar with real byte-level tracking via XHR
- Spinning icon while uploading, green check on success, red on error
- Overall progress bar and file counter in the footer
- Auto-hides 4 seconds after all uploads complete
- Dismiss button to minimise the toast
- Works for both file and folder uploads
- i18n keys added to all 8 locale files
- Service worker cache bumped to v3

static/index.html

@@ -284,5 +284,22 @@
             <button class="about-close-btn" id="about-close-btn" data-i18n="actions.close">Close</button>
         </div>
     </div>
+
+    <!-- Upload Progress Toast -->
+    <div class="upload-toast" id="upload-toast">
+        <div class="upload-toast-header">
+            <span class="upload-toast-title" id="upload-toast-title">Uploading...</span>
+            <button class="upload-toast-close" id="upload-toast-close" title="Minimize">&minus;</button>
+        </div>
+        <div class="upload-toast-body" id="upload-toast-body">
+            <!-- File entries will be added dynamically -->
+        </div>
+        <div class="upload-toast-footer">
+            <div class="upload-toast-overall-bar">
+                <div class="upload-toast-overall-fill" id="upload-toast-overall-fill"></div>
+            </div>
+            <span class="upload-toast-stats" id="upload-toast-stats"></span>
+        </div>
+    </div>
 </body>
 </html>