security: sanitize error messages to prevent info disclosure

[cfsmp3]

Summary

• Replace detailed error messages with generic ones to prevent information disclosure
• Log actual errors for debugging while showing safe messages to users

Security Issue Addressed

Debug Information Disclosure (Low) - Previously, detailed error messages (including internal error details) were returned to users. This could leak sensitive information about the system, libraries used, or internal structure.

Changes

• backend/controllers/app_handlers.go:

  • OAuth errors: Show "Authentication failed" instead of err.Error()
  • Session errors: Show "Session error" instead of err.Error()
  • Logout errors: Show "Logout failed" instead of err.Error()
  • Log actual errors for debugging

• backend/controllers/add_task.go:

  • JSON decode errors: Show "Invalid request body" instead of decode error details

Notes

• Validation errors (like "Invalid date format", "Invalid dependencies") are intentionally kept as they help users understand what needs to be fixed
• This is a focused fix for the most critical error exposures; additional controllers could be updated similarly in follow-up PRs

Test plan

• Verify OAuth errors show generic message
• Verify session errors show generic message
• Verify detailed errors are logged server-side
• Verify validation errors still provide helpful feedback

🤖 Generated with Claude Code

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