Fix react-toastify Sass @import deprecation warnings

Warning fixed:
- "Sass @import rules are deprecated" emitted from "react-toastify/scss/main.scss" during yarn build.

Approach:
- Stop compiling Toastify Sass source and import the packages compiled CSS (react-toastify/dist/ReactToastify.css) instead.
- Remove the upstream SCSS @use from InternalStyles.scss.

Why this approach:
- Preserves Toastify styling behavior while eliminating upstream Sass deprecation output in our build.
- Avoids carrying or maintaining local forks of third-party styles.

Alternatives considered:
- Keep importing SCSS and suppress warnings globally: rejected because it hides actionable deprecations.
- Patch Toastify Sass locally: rejected due to maintenance overhead and divergence from upstream.

Co-authored-by: Chris Zetter <zetter-rpf@users.noreply.github.com>

Author: cursoragent

src/assets/stylesheets/ExternalStyles.scss

@@ -3,5 +3,6 @@
 @use "../../../node_modules/react-toggle/style.css";
 @use "../../../node_modules/prismjs/plugins/line-numbers/prism-line-numbers.css";
 @use "../../../node_modules/prismjs/plugins/line-highlight/prism-line-highlight.css";
+@use "../../../node_modules/react-toastify/dist/ReactToastify.css";
 @use "../../../node_modules/material-symbols/sharp.scss";
 @use "../../../node_modules/plotly.js/src/css/style.scss" as plotlyStyle;

src/assets/stylesheets/InternalStyles.scss

@@ -1,5 +1,3 @@
-@use "../../../node_modules/react-toastify/scss/main.scss" as *;
-
 @use "./rpf_design_system/colours" as *;
 @use "./rpf_design_system/font-size" as *;
 @use "./rpf_design_system/font-weight" as *;