fix(studio): use warning tokens for previously used JWT signing key badge (#45460)

## Summary
- Replaces invalid Tailwind purple classes on the "previously used" JWT
key badge with the design system's warning tokens, matching the
bg-*-200/text-*-600/border-*-500 pattern used by the in_use and revoked
variants
- text-purple-1200 didn't exist (Tailwind purple scale stops at 950) and
the design system has no purple palette, which is why the badge rendered
with off-theme default Tailwind purple

Resolves
[FE-3128](https://linear.app/supabase/issue/FE-3128/fix-previous-key-button-color-in-jwt-rotation-modal)

## Test plan
- [x] Open the Rotate JWT signing key modal — "Previous key" badge
should match the warning palette and visually fit alongside
Standby/Current
- [x] JWT signing keys table row with status `previously_used` renders
with the same warning styling

## Example 
<img width="426" height="570" alt="image"
src="https://github.com/user-attachments/assets/34baca2b-48c0-4cd3-b997-d5c029182ad4"
/>

apps/studio/components/interfaces/JwtSecrets/jwt.constants.ts

@@ -12,7 +12,7 @@ export const statusLabels: Record<JWTSigningKey['status'], string> = {
 export const statusColors: Record<JWTSigningKey['status'], string> = {
   standby: 'bg-surface-300 text-foreground border border-foreground-muted',
   in_use: 'bg-brand-200 text-brand-600 border-brand-500',
-  previously_used: 'bg-purple-300 dark:bg-purple-100 text-purple-1200 border-purple-800',
+  previously_used: 'bg-warning-200 text-warning-600 border-warning-500',
   revoked: 'bg-destructive-200 text-destructive-600 border-destructive-500',
 }