fix(feedback): Fix form width on mobile devices (getsentry#13068)

On mobile devices, the form looked narrow due to the unspecified value
of the css variable --form-width. I set it to 100% for mobile devices
and this stretched the form to the full width (see before/after
screenshots).

before:
<img width="320" alt="before"
src="https://github.com/user-attachments/assets/60cb8741-02e4-4845-804b-a121861b4ae5">

after:
<img width="320" alt="after"
src="https://github.com/user-attachments/assets/645d8ba3-d694-432c-9c1d-2f3d939b6ccd">

---------

Co-authored-by: Catherine Lee <[email protected]>

Author: artzhookov

Diff for: packages/feedback/src/modal/components/Dialog.css.ts

@@ -100,7 +100,7 @@ const FORM = `
 }
 
 .form__right {
-  flex: 0 0 var(--form-width, 272px);
+  flex: 0 0 auto;
   width: var(--form-width, 272px);
   display: flex;
   overflow: auto;
@@ -111,7 +111,7 @@ const FORM = `
 
 @media (max-width: 600px) {
   .form__right {
-    width: auto;
+    width: var(--form-width, 100%);
   }
 }