Fix user webmail SSO entrypoints

[zylos01]

Summary

This fixes two related issues in the user webmail SSO flow:

1. The user-facing Webmail entrypoint currently links directly to /SOGo/so, which bypasses the intended SSO handoff path.
2. sogo-auth.php can reject a mailbox user's own SSO login because it only accepts alias-based lookup in paths where the mailbox itself should also be valid.

Changes

• Change the user webmail link target to /sogo-auth.php?login=%u
• Change the user auth tab button to use /sogo-auth.php?login={{ mailcow_cc_username }}
• In sogo-auth.php, allow the currently logged-in mailbox user through the same SSO path without requiring alias lookup
• In the authenticated SOGo path, treat the current mailbox user as valid even if user_get_alias_details() is not the matching mechanism
• Fix the header link substitution guard to check the processed app entry consistently before replacing %u

Why

In our reproduction, mailbox users could hit Unauthorized / broken webmail navigation because the user-facing entrypoint skipped the SSO helper, and the SSO helper itself was stricter than necessary for the mailbox owner case.

Validation

I was able to reproduce the broken user webmail flow and confirm that these changes restore the expected SSO navigation path.

I could not run local PHP linting in this environment because php is not installed here.

[milkmaker]

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