Add browser callback/code authentication (WSO2/CAS) as alternative to password

[MattXcz]

Motivation

• Add support for browser callback code/ticket or full callback URL so users who sign in via SSO (WSO2 Identity Server / Google/Apple) can authenticate without storing a password.
• Keep existing password-based login while allowing a fallback for service-ticket style authentication.
• Improve UX by exposing the new option in the config flow, translations and README.

Description

• Introduced new config keys CONF_BROWSER_AUTH and CONF_SERVICE_TICKET and exposed them in const.py.
• Extended the config flow (config_flow.py) to accept an optional browser_auth field, validate mutual exclusivity between password and browser callback input, and persist browser_auth (also saved as service_ticket for backwards compatibility).
• Updated the API client (api.py) to accept browser_auth, added _login_with_browser_auth which accepts either a raw code/ticket or a full callback URL and exchanges it for an API token, and added _extract_browser_auth_value to parse the provided input.
• Adjusted integration setup (__init__.py) to read browser_auth (or legacy service_ticket) from the config entry and pass it to the API client.
• Updated user-facing text in README.md and translations (translations/cs.json and translations/en.json) to document the new authentication method and to add validation error messages (missing_auth_data, too_many_auth_methods).

Testing

• No automated tests were added or executed as part of this change.

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Codex Task