Fix material-symbols Sass deprecations by using CSS asset

Warnings fixed:
- "Sass @import rules are deprecated" from material-symbols/sharp.scss.
- "Global built-in functions are deprecated" (to-lower-case, str-index, str-slice, str-length) from material-symbols/_core.scss.

Approach:
- Replace material-symbols/sharp.scss import with material-symbols/sharp.css in ExternalStyles.scss.

Why this approach:
- Uses upstream distributed CSS, so our build no longer compiles third-party Sass that triggers deprecation warnings.
- Keeps the same font-face and utility class output with minimal risk.

Alternatives considered:
- Keep SCSS and silence deprecations: rejected because it masks dependency issues broadly.
- Patch material-symbols Sass locally: rejected due to long-term maintenance burden.

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

Author: cursoragent

src/assets/stylesheets/ExternalStyles.scss

@@ -4,5 +4,5 @@
 @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/material-symbols/sharp.css";
 @use "../../../node_modules/plotly.js/src/css/style.scss" as plotlyStyle;