[BUG] Zip download missing extension, silent fail on empty #523

Describe the bug
Zip download lacks file extension and shows success even when download fails entirely.

To Reproduce

1. Go to Catalogue page
2. Select 2+ papers
3. Click "Download Selected"
4. (Optional: throttle network in DevTools to simulate all requests failing)
5. File downloads with no .zip extension
6. If all fetches failed, empty ZIP is created but success toast still appears

Expected behavior

• Downloaded file should have .zip extension (e.g., Physics.zip)
• If all paper fetches fail, show error toast instead of success
• Success message should show count: "Downloaded 3 of 5 papers"

Screenshots
N/A (UX issue — filename shows Physics not Physics.zip in downloads folder)

Desktop

• OS: Any
• Browser: Chrome/Firefox/Safari
• Issue occurs on all browsers

Root cause
File: src/context/filterContext.tsx

• Line 180: a.download = getDownloadName(...) missing .zip
• Lines 150–164: Fetch errors silently caught, no success counter
• Line 186: Always shows success toast regardless of download state

Additional context
Low severity — UX friction, not data loss. User gets empty ZIP but can retry.