feat: clean up stylesheets

[oliverlaz]

💡 Overview

The SDK stylesheet had a lot of icons used in our sample apps, unnecessarily increasing the size of the shipped CSS.
This PR moves those icons to the appropriate sample applications and trims around 50KB of SDK CSS.

As part of this refactor, a few more improvements are made:

• vendored css properties are sorted correctly, so they don't take precedence
• updates to the latest sass version

[Copilot]

Pull Request Overview

The PR refactors sample-app stylesheet imports to use Sass modules, scopes breakpoint mixins under a breakpoints namespace, extracts icon definitions into the sample app, and bumps the Sass dependency to v1.89.2.

• Converted all @import statements to @use and updated mixin calls to breakpoints.respond-above(...)
• Moved icon mask definitions from the SDK CSS into the React sample-app’s icons.scss
• Upgraded Sass version and modernized utility mixin property order

Reviewed Changes

Copilot reviewed 62 out of 63 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
style/index.scss	Swapped @import for @use for all partials
style/breakpoints.scss	Added @use 'sass:map' and updated map API calls
style/icons.scss	Added large set of icon mask definitions locally
packages/styling/src/_utils.scss	Modernized apply-mask-image mixin declarations
sample-apps/react/messenger-clone/src/styles/index.scss	Replaced CSS imports with meta.load-css and updated vendor CSS paths

Comments suppressed due to low confidence (2)

sample-apps/react/messenger-clone/src/styles/index.scss:9

• [nitpick] The meta.load-css calls mix extension-less and .scss file names; unify usage (either always include .scss or omit it) to avoid import resolution confusion.

@include meta.load-css('layout');

sample-apps/react/messenger-clone/src/styles/index.scss:3

• Double-check that the path 'stream-chat-react/css/v2/index.css' resolves correctly under Sass module resolution; if it fails, consider using the npm package import syntax (e.g. ~stream-chat-react/css/v2/index.css).

  @import 'stream-chat-react/css/v2/index.css';