[BUG] Reverting denial of canvas permissions

### Confirmation Checklist - [x] I confirm that this feature is not already present on the **latest release** of Phoenix. You can check what the latest version is on [the `Releases` page](https://codeberg.org/celenity/Phoenix/releases). - [x] I confirm that this feature has **NOT** already been suggested on [the Codeberg issue tracker](https://codeberg.org/celenity/Phoenix/issues), [the GitLab issue tracker](https://gitlab.com/celenityy/Phoenix/-/issues), **and/or** [the GitHub issue tracker](https://github.com/celenityy/Phoenix/issues). ### Please explain your proposal with as many details as necessary (Ex. what you're suggesting, why you're suggesting it, what need you thinks it will fill, who it will benefit, etc...). Hi, I love this codething, thanks for creating it. To the best of my knowledge, it seems either impossible or excessively difficult to revoke previously blocked `Read canvas contents` site permissions (see pic below). <img width="389" height="199" alt="Image" src="https://github.com/user-attachments/assets/9c110bef-051c-4eb9-a9d5-1b72f9ee675e" /> <br> <br> ### The option to modify this permission would usually be located here: <img width="400" height="500" alt="Image" src="https://github.com/user-attachments/assets/19cda441-feb3-4af2-973a-dd65bc635234" /> <br> <br> I also looked through `about:config`, Phoenix wiki and the regular firefox settings, but could not locate a way to revert said permission preferences in any place once it's already denied. Thanks in advance.