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/it.json

@@ -16,6 +16,8 @@
  "upload": "Carica",
  "upload_files": "Carica file",
  "upload_folder": "Carica cartella",
+ "upload.uploading": "Caricamento...",
+ "upload.complete": "{count} / {total} caricati",
  "rename": "Rinomina",
  "move": "Sposta in...",
  "move_to": "Sposta in",
@@ -311,7 +313,7 @@
  "fa": "Persiano",
  "fr": "Francese",
  "de": "Tedesco",
- "pt": "Portoghese",
+ "pt": "Portoghese"
  "it": "Italiano"
  }
  },