As discussed on Matrix, Vikunja currently prevents users from using LDAP
authentication if the server allows anonymous binds (common in local
environments like YunoHost). The application would previously trigger a
`log.Fatal` if `AuthLdapBindDN` or `AuthLdapBindPassword` were left
empty in the configuration.
#### **How this fixes the problem:**
* **Validation:** Removed the strict requirement for Bind credentials in
`InitializeLDAPConnection`.
* **Connection Logic:** Updated `ConnectAndBindToLDAPDirectory` to
attempt an `UnauthenticatedBind` from the `go-ldap` library when no
credentials are provided.
* **Safety:** If a Bind DN is provided, the behavior remains unchanged
(authenticated bind).
#### **Testing:**
* Tested manually on a **YunoHost** instance by replacing the binary.
* Confirmed that Vikunja now successfully starts and authenticates users
via the local LDAP (localhost) without requiring a service account.
* Added a basic unit test in `pkg/modules/auth/ldap/ldap_test.go` to
ensure the initialization logic doesn't crash with empty credentials.
*Note: This is my first contribution to a Go project (assisted by an LLM
for syntax). Feedback on code style is more than welcome!*
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