fix(mobile): handle base_not_on_remote in PR-create block message (#8676)

#8651 added 'base_not_on_remote' to HostedReviewCreationBlockedReason but
did not add a matching case to getMobilePrCreateBlockMessage, leaving the
switch non-exhaustive so the function can fall through and implicitly
return undefined against its string | null type. That fails `tsc` on main
(verify does not run on direct pushes, so it surfaced on PR merge commits).
Add the missing case with actionable copy consistent with the desktop
create-time message.

mobile/src/source-control/mobile-pr-create.ts

@@ -66,6 +66,8 @@ export function getMobilePrCreateBlockMessage(prefill: MobilePrPrefill): string
       return `A ${copy.reviewLabel} already exists for this branch.`
     case 'fork_head_unsupported':
       return `Creating a ${copy.reviewLabel} from this fork is not supported.`
+    case 'base_not_on_remote':
+      return `Push the base branch before creating a ${copy.reviewLabel}.`
     case 'needs_push':
     case null:
     case undefined: