feat: #1007 - Option to disable local login when OIDC is enabled

[DerDummePunkt]

This PR attempts to implement #1007

Adds configurable local authentication through GOTIFY_LOCALAUTH_ENABLED, defaults to true to preserve existing behavior.

When local authentication is disabled:

• The server does not register POST /auth/local/login.
• Basic Auth with local Gotify user credentials is rejected by authentication middleware.
• The check for a local admin account and subsequent creation is skipped.
• The UI hides the local username/password login form, registration button, and password-based elevation form.
• The UI receives the public localauth flag through injected window.config.
• GET /gotifyinfo exposes the same flag as localauth.

The server exits with a fatal error when both local authentication and OIDC are disabled, preventing startup without an available login method.

[jmattheis]

Looks good so far. I've some remarks and haven't tested it manually.

[jmattheis]

I've made some small fixes.

I'll test this some more and will merge it around end of next week. (For security related stuff I want to take my time and I'm currently a bit busy).

[DerDummePunkt]

No worries, take your time, there's no rush

[codecov]

Codecov Report

❌ Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.63%. Comparing base (491a17a) to head (795d9d1).

Files with missing lines	Patch %	Lines
app.go	0.00%	1 Missing ⚠️
router/router.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1020      +/-   ##
==========================================
+ Coverage   74.28%   74.63%   +0.34%     
==========================================
  Files          66       66              
  Lines        3512     3532      +20     
==========================================
+ Hits         2609     2636      +27     
+ Misses        698      690       -8     
- Partials      205      206       +1     

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[jmattheis]

Thanks