AGH: {{log.enabled: false}} leaves the legacy logger enabled

### Prerequisites

- [x] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) and found no answer
- [x] I have searched other issues and found no duplicates
- [x] I want to report a bug and not ask a question or ask for help
- [x] I have set up AdGuard Home correctly and configured clients to use it

I found this while investigating the macOS service-log behavior in #8564. I
use and really like AdGuard Home on my Mac, so I followed the logging path a
little further and noticed this separate, platform-independent regression.

I am filing it separately because it is a small bug with a small fix, and it
does not belong in the larger macOS logging change.

### Plain English

AdGuard Home is moving from an older logger to a newer one. Setting
`log.enabled: false` correctly silences the newer logger, but the older logger
can remain switched on. The fix makes the same off switch control both.

### Platform (OS and CPU architecture)

All platforms. Reproduced from a source checkout on macOS ARM64.

### Installation

Source checkout of current `master`.

### Setup

Focused logger regression test; the defect is platform-independent.

### AdGuard Home version

Current `master` at `090f75d79fcad19a7447d158fc926a304b0dff57`.

### Action

Load logging settings with:

```yaml
log:
enabled: false
```

Then construct the configured logger. The focused unchanged-code regression
sets the transitional legacy logger to `INFO`, calls `newSlogLogger` with
`Enabled: false`, and checks its resulting level.

### Expected result

Both the structured logger and the transitional
`github.com/AdguardTeam/golibs/log` logger are disabled, matching the feature
introduced for #7079. The legacy logger level should be `OFF`.

### Actual result

`newSlogLogger` returns the discard structured logger before reaching the
existing `log.SetLevel(log.OFF)` branch. The structured logger is disabled,
but the legacy logger remains at its previous level and may continue emitting.

The unchanged-code regression produced:

```text
expected legacy log level: OFF
actual legacy log level: INFO
```

### Additional information and/or screenshots

I have a two-line fix ready that moves `log.SetLevel(log.OFF)` into the disabled
early-return path and removes the unreachable duplicate condition. It is
isolated on a one-commit branch with a changelog entry and regression test.

Proof after the fix:

```sh
go test -race ./internal/home -run '^TestNewSlogLoggerDisabled$' -count=20
```

The focused test passed 20 consecutive runs, and the full `make go-check`
suite passed. This change is intentionally separate from the proposed macOS
service-logging enhancement.