BUG: Buffer boundary not validated in MIME parsing

[andrinoff]

Describe the bug
MIME boundary extraction at fetcher/fetcher.go:1120 doesn't validate endIdx > 0 before accessing endIdx-1. Boundary at position 0 causes panic.

To reproduce

1. Send email with malformed MIME boundary at position 0
2. Access rawEmail[endIdx-1] when endIdx=0
3. Index out of bounds panic

Expected behavior
Check endIdx > 1 before accessing endIdx-1

Matcha version
master

OS
All

Additional context
File: fetcher/fetcher.go line 1120
Edge case in malformed MIME