Unite icons (#1374)

## Launch Checklist

This removes the `@mdi` (material design icons) package and uses the
icons from `react-icons`.
The icons are not exactly the same, but have the same idea behind them.

 - [x] Briefly describe the changes in this PR.
- [x] Include before/after visuals or gifs if this PR includes visual
changes.
 - [x] Add an entry to `CHANGELOG.md` under the `## main` section.

Main changes can be found below.
Before:
<img width="580" height="780" alt="image"
src="https://github.com/user-attachments/assets/e3d5fc8a-bd59-48fe-bdae-31bb290749c8"
/>


After:

<img width="580" height="780" alt="image"
src="https://github.com/user-attachments/assets/bacdbdba-9e73-4bef-bd30-26fe946269c1"
/>

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/components/_FunctionButtons.tsx

@@ -2,7 +2,7 @@ import React from "react";
 
 import InputButton from "./InputButton";
 import {MdFunctions, MdInsertChart} from "react-icons/md";
-import {mdiFunctionVariant} from "@mdi/js";
+import { TbMathFunction } from "react-icons/tb";
 import { type WithTranslation, withTranslation } from "react-i18next";
 
 type FunctionInputButtonsInternalProps = {
@@ -24,9 +24,7 @@ class FunctionInputButtonsInternal extends React.Component<FunctionInputButtonsI
           onClick={this.props.onExpressionClick}
           title={t("Convert to expression")}
         >
-          <svg style={{width:"14px", height:"14px", verticalAlign: "middle"}} viewBox="0 0 24 24">
-            <path fill="currentColor" d={mdiFunctionVariant} />
-          </svg>
+          <TbMathFunction />
         </InputButton>
       );