chore(1-3356): make mui buttons respect font size changes (#9275)

Makes the MUI buttons use `body1.fontSize` for their own font size.

Turns out we had a hardcoded `16px` in the `app.css` file, which made it
hard to override. This change removes mui button styling in `app.css`
and puts it into the `theme.ts` file instead.

Also updates the border radius to match the one we use in the theme
(from 3px to 4px). It's a subtle change:

before:

![image](https://github.com/user-attachments/assets/a6f94250-9b4b-43e0-aa65-280f3fa007bb)


after:

![image](https://github.com/user-attachments/assets/3ef205c8-8119-4bf1-9541-39371b26dda9)

frontend/src/themes/app.css

@@ -27,12 +27,6 @@ button {
     margin: 0;
 }
 
-.MuiButton-root {
-    border-radius: 3px;
-    text-transform: none;
-    font-size: 16px;
-}
-
 .skeleton {
     position: relative;
     overflow: hidden;
@@ -161,7 +155,7 @@ input.hide-clear[type="search"]::-webkit-search-results-decoration {
 
 /*
     This style is to hide the error message block from "vanilla-jsoneditor" package.
-    The package has another minmal way to show error inline with the editor. And so we are hiding this block 
+    The package has another minimal way to show error inline with the editor. And so we are hiding this block
     which shows at the bottom of the editor and does not have the best UX - because it shows errors even as user
     is typing in the editor.
 */

frontend/src/themes/theme.ts

@@ -18,7 +18,10 @@ export const baseTheme = {
         fontWeightBold: '700',
         fontWeightMedium: '700',
         allVariants: { lineHeight: 1.4 },
-        button: { lineHeight: 1.75 },
+        button: {
+            fontSize: `${15 / 16}rem`,
+            lineHeight: 1.75,
+        },
         h1: {
             fontSize: '1.5rem',
             lineHeight: 1.875,
@@ -332,6 +335,16 @@ export const lightTheme = createTheme({
             },
         },
 
+        // Buttons
+        MuiButton: {
+            styleOverrides: {
+                root: ({ theme }) => ({
+                    borderRadius: theme.shape.borderRadius,
+                    textTransform: 'none',
+                }),
+            },
+        },
+
         // Links
         MuiLink: {
             styleOverrides: {