fix: handle empty messages in login error and warn about special characters

[kwaczek]

Summary

Fixes #63

• When login fails and the API returns an empty messages array, the integration crashed with IndexError: list index out of range. This is now handled gracefully.
• When login fails with 401 and no error message from the server, the user now gets a helpful hint: "If your password contains special characters (e.g. `, $, +, etc.), try changing it to use only standard characters — Rohlik.cz may not handle them correctly."
• Added a note to the README under Configuration warning about this known server-side limitation.

Root cause

Rohlik.cz server doesn't properly handle certain special characters in passwords (e.g. backtick `). When such a password is sent, the server returns a 401 response with an empty messages array instead of the usual error message. The original code assumed messages would always have at least one element.

Test plan

• Verified fix handles empty messages array without crashing
• Verified helpful error message is shown for 401 with empty messages
• Verified normal 401 flow (wrong password, non-empty messages) still works as before
• Verified non-401 errors with empty messages also handled correctly

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes a crash in the Rohlik.cz Home Assistant integration when the login API returns an empty messages array, and adds user-facing guidance about password special-character login failures.

Changes:

• Hardened login error handling to avoid IndexError when messages is empty and to provide a fallback error detail.
• Added a README configuration note warning that certain special characters in passwords may cause login failures.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
custom_components/rohlikcz/rohlik_api.py	Safely handles empty messages in login failure responses and improves fallback error messaging/logging.
readme.md	Adds a configuration note about potential login issues with special characters in passwords.

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