Spellcheck should be disabled for filtering rules textarea

### 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](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a)

- [X] I have set up AdGuard Home correctly and [configured clients to use it](https://github.com/AdguardTeam/AdGuardHome/wiki/Clients). (Use the [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) for help with installing and configuring clients.)

### Platform (OS and CPU architecture)

Linux, AMD64 (aka x86_64)

### Installation

Custom package (OpenWrt, HomeAssistant, etc; please mention in the description)

### Setup

On a router, DHCP is handled by the router

### AdGuard Home version

0.107.26

### Action

I opened the Custom Filtering Rules page and found that Firefox highlights most of it in a red underline,
which suggests that its spell-checking features are being utilized for that form.

This does not inhibit usage of the textarea, but makes it quite annoying to read,
as it is littered with red underlines.

### Expected result

[...]

### Actual result

As custom filtering rules mostly contain technically non-correct English (`dnsrewrite, etc.),
the spell-checking features should be disabled for its respective `textarea`.

This change would make it a lot easier to read, and makes the page look nicer.

### Additional information and/or screenshots

See [the relevant MDN article on the `spellcheck` attribute](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/spellcheck).