BUG: Signed/unsigned comparison in email cursor calculation

[andrinoff]

Describe the bug
Cursor calculation at fetcher/fetcher.go:407 mixes uint32 and int. Type coercion causes incorrect offset with large values.

To reproduce

1. Pass large limit value (> max int on 32-bit)
2. chunkSize cast to uint32 overflows
3. Wrong from position calculated

Expected behavior
Use consistent types or validate range

Matcha version
master

OS
32-bit systems primarily

Additional context
File: fetcher/fetcher.go line 407
Type safety issue