Fluent: add dedicated loader animation

[OEvgeny]

Changelog Entry

Added

• Added dedicated loading animation for messages in preparing state for Fluent theme, in PR #5423, by @OEvgeny

Description

This PR introduces a dedicated loading animation for messages in the preparing state in the Fluent theme.

Design

• Uses SVG-based animation with gradient colors that match Fluent theme
• Implements loader through decorator middleware and dedicated composer

Specific Changes

• Added new ActivityLoader component with custom SVG animation
• Created CSS modules for loader styling and positioning
• Extended ActivityDecorator styles to handle loader presentation
• Added decorator middleware to render loader during "preparing" livestream state
• Integrated loader with DecoratorComposer in FluentThemeProvider

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[compulim]

Nitpicking.