Fixed visual bug with extracted assignment count and fixed first stage of redundant deduplication logic

Fixed issue where the .ics file export feature would fail in the background on Firefox

Added .ics file export support

More robust race condition handling when dealing with auth tokens, which should prevent a visual bug where calendars seemed disconnected in the popup but remained connected in the options page. Also added an identifier in the options page for sync type.

I just updated the png logos (like the one that appears in the taskbar)

Known Visual Bug

After first connecting to Google Calendar, extracted assignments sync automatically, but the popup doesn't update immediately. If left open, the popup refreshes 5-10 seconds after sync completes (even though calendar events are created instantly).
Workaround: Close and reopen the popup to see updated status immediately.
The sync itself works correctly—this is purely a visual delay in the popup UI. Will be looking into fixes for next version.