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/css/style.css

@@ -3472,4 +3472,133 @@ html[dir='rtl'] .fa-arrow-left::before {
 html[dir='rtl'] .fa-sign-out-alt {
     -webkit-transform: rotate(180deg);
     transform: rotate(180deg);
+}
+
+/* ============================================================================
+   Upload Progress Toast
+   ============================================================================ */
+.upload-toast {
+    position: fixed;
+    bottom: 24px;
+    right: 24px;
+    width: 360px;
+    max-height: 400px;
+    background: #fff;
+    border-radius: 12px;
+    box-shadow: 0 8px 30px rgba(0,0,0,0.15);
+    z-index: 10000;
+    display: none;
+    flex-direction: column;
+    overflow: hidden;
+    font-family: inherit;
+    animation: uploadToastSlideIn 0.3s ease-out;
+}
+.upload-toast.visible {
+    display: flex;
+}
+@keyframes uploadToastSlideIn {
+    from { transform: translateY(20px); opacity: 0; }
+    to   { transform: translateY(0);    opacity: 1; }
+}
+.upload-toast-header {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    padding: 12px 16px;
+    background: #ff5e3a;
+    color: #fff;
+}
+.upload-toast-title {
+    font-weight: 600;
+    font-size: 14px;
+}
+.upload-toast-close {
+    background: none;
+    border: none;
+    color: #fff;
+    font-size: 20px;
+    cursor: pointer;
+    line-height: 1;
+    padding: 0 4px;
+    opacity: 0.8;
+}
+.upload-toast-close:hover { opacity: 1; }
+
+.upload-toast-body {
+    flex: 1;
+    overflow-y: auto;
+    padding: 8px 0;
+    max-height: 260px;
+}
+.upload-toast-file {
+    display: flex;
+    align-items: center;
+    padding: 6px 16px;
+    gap: 10px;
+}
+.upload-toast-file-icon {
+    font-size: 16px;
+    color: #999;
+    flex-shrink: 0;
+    width: 20px;
+    text-align: center;
+}
+.upload-toast-file-icon.done { color: #34c759; }
+.upload-toast-file-icon.error { color: #ff3b30; }
+.upload-toast-file-info {
+    flex: 1;
+    min-width: 0;
+}
+.upload-toast-file-name {
+    font-size: 13px;
+    color: #333;
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+}
+.upload-toast-file-bar {
+    height: 3px;
+    background: #eee;
+    border-radius: 2px;
+    margin-top: 4px;
+    overflow: hidden;
+}
+.upload-toast-file-fill {
+    height: 100%;
+    background: #ff5e3a;
+    width: 0%;
+    transition: width 0.2s ease;
+    border-radius: 2px;
+}
+.upload-toast-file-fill.done { background: #34c759; }
+.upload-toast-file-fill.error { background: #ff3b30; }
+.upload-toast-file-pct {
+    font-size: 12px;
+    color: #999;
+    flex-shrink: 0;
+    width: 38px;
+    text-align: right;
+}
+
+.upload-toast-footer {
+    padding: 10px 16px;
+    border-top: 1px solid #f0f0f0;
+}
+.upload-toast-overall-bar {
+    height: 4px;
+    background: #eee;
+    border-radius: 2px;
+    overflow: hidden;
+    margin-bottom: 6px;
+}
+.upload-toast-overall-fill {
+    height: 100%;
+    background: #ff5e3a;
+    width: 0%;
+    transition: width 0.25s ease;
+    border-radius: 2px;
+}
+.upload-toast-stats {
+    font-size: 12px;
+    color: #888;
 }