Refactor Slack Chrome cookie decryption to use shared browser cookies package

[priyanshujain]

Summary

After the browser cookie extraction package is added at internal/browser/cookies/, the Slack desktop cookie extraction code at source/slack/desktop/cookies.go should be refactored to use the shared Chrome decryption functions instead of having its own copy.

Current state

source/slack/desktop/cookies.go has its own Chrome cookie decryption:

• getKeychainPassword() - macOS Keychain access
• decryptCookie() - AES-128-CBC with PBKDF2 key derivation
• copyToTemp() - temp file copy to avoid browser locks

The new internal/browser/cookies/chrome.go package will have the same logic as a shared utility.

What to do

• Update source/slack/desktop/cookies.go to import and use internal/browser/cookies
• Remove the duplicated decryption functions from the Slack package
• Keep Slack-specific logic (cookie paths, host filtering, xoxd token extraction)
• Verify existing Slack tests still pass