Login fails with password containing non-ascii characters

## About the bug
just set up influxdb and spent way to much time trying to figure out why i cant log in. After trying a lot of random bullshit, i noticed that the base64 in the Basic authentication header looked different in my browser than when I use curl.

(i'm fine with this password being public, i've changed it and the server isn't publicly accessible anyways)
Curl:
>Authorization: Basic aW5mbHV4OkJyYW5kRWluaWdlckd1dGVzRXJuc3RBdXNnZWzDtnN0

```
$ echo "aW5mbHV4OkJyYW5kRWluaWdlckd1dGVzRXJuc3RBdXNnZWzDtnN0" | base64 -d
>influx:BrandEinigerGutesErnstAusgelöst%
```
FIrefox:
>authorization: Basic aW5mbHV4OkJyYW5kRWluaWdlckd1dGVzRXJuc3RBdXNnZWz2c3Q=

```
$ echo "aW5mbHV4OkJyYW5kRWluaWdlckd1dGVzRXJuc3RBdXNnZWz2c3Q=" | base64 -d
>influx:BrandEinigerGutesErnstAusgel�st%
```

**Steps to reproduce:**
1. create an user with a password containing non-ascii unicode characters
2. try to log in

**Expected behavior:**
login succeeds

**Actual behavior:**
recieve 401: unauthorized access

**Visual Proof:**


## About your environment

**Environment info:**
influx 2.7.5 docker container

**Config:**
default config

Start by reproducing the login flow from the listed steps with the provided non-ASCII password, comparing the browser request with curl's Basic authentication header. Trace the UI authentication entry point and verify that login succeeds and returns no 401 for such passwords; add a regression check if the existing authentication tests cover this flow.

Written by the indexing model from the issue text.