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/locales/de.json

@@ -16,6 +16,8 @@
     "upload": "Hochladen",
     "upload_files": "Dateien hochladen",
     "upload_folder": "Ordner hochladen",
+    "upload.uploading": "Wird hochgeladen...",
+    "upload.complete": "{count} / {total} hochgeladen",
     "rename": "Umbenennen",
     "move": "Verschieben nach...",
     "move_to": "Verschieben nach",