Fluent: add attachments bar

[OEvgeny]

Fixes #

Changelog Entry

• Resolved #5463. Added attachment preview for sendAttachmentOn: "send", in PR #5464, by @compulim, in PR #5492, by @OEvgeny
  • Attaching files will no longer remove previously attached files
  • Updated Fluent theme to use the new attachment preview feature

Description

This implements the new attachments bar for Fluent skin pack.

Design

The original component is exported and reused in the fluent-theme package. Styles are adjusted to use Fluent design system variables.

Specific Changes

• Updated Fluent theme attachments implementations
• Removed unused components from Fluent theme package
• Updated existing tests: added a test condition for Fluent theme
• Removed old tests as they're not compatible (2 tests removed in total)
• Fixed lint error in ComponentIcon component
• Added a new global handler to render WebChat or WebChat with Fluent skin pack based on theme and variant query parameter

-

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

Review Checklist

This section is for contributors to review your work.

• 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.