## Problem
When users clicked on a plan card in the Cal.com Platform pricing UI, the loading
spinner would appear on ALL plan cards simultaneously instead of just the selected one.
This gave a confusing UX impression that all plans were being processed at the same time.
## Solution
Track the currently loading plan using a `loadingPlan` state variable that stores
which plan the user clicked on, rather than relying solely on the pending state from
the mutation hook.
## Changes
- Added `loadingPlan` state to track which specific plan is loading
- Update `loadingPlan` when a plan subscription is initiated
- Reset `loadingPlan` on error to ensure UI returns to normal state
- Modified `isLoading` prop condition to check if the current plan matches
the `loadingPlan` before showing the spinner
This ensures that only the plan card the user clicked on displays the loading indicator,
while other plans remain in their normal state.
