feat(permissions) : add role-based UI guards and readonly mode for viewers

- Add usePermissions composable (isAdmin, canEdit, canView)
- Password-protected profile login with modal on profiles page
- Disable all form fields for ROLE_VIEWER across edit/create pages
- Show navigation buttons (Modifier/Consulter) for all roles, hide delete for viewers
- Add readonly prop to ModelTypeForm for category pages
- Disable modal fields (sites, constructeurs) for viewers
- Guard /admin routes in middleware

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/composables/useApi.ts

@@ -66,7 +66,9 @@ export function useApi() {
           const text = await response.text().catch(() => '')
           errorData = text ? { message: text } : {}
         }
-        const errorMessage = (errorData.message as string) || `Erreur ${response.status}: ${response.statusText}`
+        const errorMessage = response.status === 403
+          ? 'Permissions insuffisantes pour cette action.'
+          : (errorData.message as string) || `Erreur ${response.status}: ${response.statusText}`
         showError(errorMessage)
         return { success: false, error: errorMessage, status: response.status }
       }