fix: Set correct font-weight to variable font (#4550)

Some `font-weight` CSS directives do not work on Cozy apps. For example,
in this screenshot from cozy-settings, "Add an additional security
level" has `font-weight: 500` set by cozy-ui but it is not taken into
account.

<img width="551" height="664" alt="Screenshot 2025-07-18 at 16 53 52"
src="https://github.com/user-attachments/assets/e311ba69-1a75-4c8c-8aa8-b3d4d52d8022"
/>

It seems that setting a range in
[@font-face](https://developer.mozilla.org/en-US/docs/Web/CSS/@font-face/font-weight#syntax)
like in this PR solves the issue. It worked by changing it directly in
Firefox developer tools but in this PR it does not.

So to summarize : 
- There is an issue with font weight
- I think it is related to how we use @font-face
- This PR does not fix it

Did I miss something ?

Author: nono

assets/fonts/Inter-VariableFont_opsz,wght.ttf

@@ -1,7 +1,7 @@
 @font-face {
   font-family: Inter;
   font-style: normal;
-  font-weight: normal;
-  src: url("../fonts/Inter-VariableFont_opsz,wght.ttf");
+  font-weight: 100 900;
+  src: url("InterVariable.woff2");
   font-display: fallback;
 }