fix(mail): sidebar background in mobile view

[GursheenK]

frappe-ui Sidebar is transparent in dark mode by design. It sits flush on surface-base and is set apart only by its border.

But mail's mobile view re-uses the same Sidebar as a fixed overlay over content, so the transparent background lets content show through. Since we add the overlay behavior, this fix forces an opaque bg-surface-base only on mobile, leaving desktop untouched.

If the design intent changes, this can move upstream to frappe-ui later. For now, keeping it in our app makes sense.

[greptile-apps]

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
frontend/src/apps/mail/components/AppSidebar.vue	Adds a mobile-only opaque background class to the sidebar overlay.

_{Reviews (1): Last reviewed commit: "fix: opaque mobile sidebar background" | Re-trigger Greptile}

[GursheenK]

@krantheman merging this for now you can take a look and remove later if having mobile-specific classes in frappe ui sidebar makes more sense eventually