BUG: Race condition in folderEmails map access without synchronization

[andrinoff]

Describe the bug

In markEmailAsReadInStores() (main.go:738-745), the code iterates over and modifies m.folderEmails map without any synchronization. This map is also accessed from goroutines in saveFolderEmailsToCache() (line 630) and fetchFolderEmailsCmd() (line 2426+), creating a data race.

To reproduce

1. Rapidly switch folders while emails are being fetched
2. Run with -race flag to detect

Expected behavior

Access to m.folderEmails should be protected by a mutex.

[floatpanebot]

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