fix: lint issues

[Aravinda-HWK]

📌 Description

This PR is to resolve the lint issues in the Raven repo.

• Closes Added golangci-lint github workflow  #112

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🔍 Changes Made

• 50 errcheck issues - Added proper error handling using _ = or defer func() { _ = ... }() patterns
• 1 ineffassign issue - Removed ineffectual assignment in internal/delivery/parser/parser.go
• 5 staticcheck issues - Fixed deprecated io/ioutil import, error message formatting, and redundant code
• 2 unused warnings - Removed unused functions (parseUIDSequenceSet and ensureDefaultMailboxes)

✅ Checklist (Email System)

• Core IMAP commands tested (LOGIN, CAPABILITY, LIST, SELECT, FETCH, LOGOUT).
• Authentication is tested.
• Docker build & run validated.
• Configuration loading verified for default and custom paths.
• Persistent storage with Docker volume verified.
• Error handling and logging verified
• Documentation updated (README, config samples).

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🧪 Testing Instructions

To test the server, use the instructions in the README in the test directory.

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📷 Screenshots / Logs (if applicable)

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⚠️ Notes for Reviewers