feat(chat): add GIF scale setting in Site Layout -> Chat

[jaissonjpp]

Proposed changes

This PR introduces a new GIF Scale setting under Site Layout -> Chat settings, allowing users to customize the display size/scale of GIFs rendered in chat messages.

Key Changes:

• HiddenElementsModule.vue: Declared the chat.gif_scale SLIDER setting under ["Site Layout", "Chat"] (range 0.1x to 1.0x, step 0.05, default 1.0x). Sets --seventv-gif-scale CSS variable on change.
• MessageTokenGif.vue: Updated the GIF image renderer to reactively bind chat.gif_scale and --seventv-gif-scale for real-time scaling of GIFs in chat messages.
• locale/en_US.yaml: Added localization labels for chat_gif_scale.
• CHANGELOG-nightly.md: Added a changelog entry for this new setting under the latest nightly release section.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING doc
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged