Improved Bulk Upload Notifications

Centralized upload progress tracking with per-item cancellation and clearer error reporting

We've completely overhauled how bulk uploads work in Papermark with a new unified notifications drawer.

Key improvements:

Centralized progress tracking — All uploads now appear in a single notification drawer with real-time progress updates
Per-item cancellation — Cancel individual uploads or stop everything with global cancellation controls
Time-left estimation — See how long remaining uploads will take
Clearer error reporting — Failed and skipped files are now grouped separately with clear rejection reasons
Download skipped files list — Export a list of files that couldn't be uploaded for review

The new system provides faster, more reliable folder uploads with improved concurrent processing and better progress reporting.