[BUG] Sale timings are incorrect on westend

Describe the bug
The calculation of the real world time of the various parts of a coretime sale are now incorrect. There was a breaking change introduced which used relay block number instead of local block number as a clock.

This just requires a change in your time calculation logic - it's still interpreting the number in storage as local block number.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://www.lastic.xyz/westend/bulkcore1
2. See incorrect time estimate (~94 months)

Expected behavior
Show the correct wall clock time for upcoming changes to the sale.

Screenshots
Now in the UI we have things like:

Platform independent

Additional context
The block number provider switch introduced in https://github.com/paritytech/polkadot-sdk/pull/5656 is live on westend with the deployment of the stable2503 release. This makes sale boundaries (interlude, lead-in, sale rotation) more deterministic in time as it is all done with the relay chain block number as clock.
It's going to hit Kusama and Polkadot with stable2503, so probably not before mid may at the earliest on Kusama and later on Polkadot.

Note
Once you take on this PR paste a comment with your hand -> ✋ so that people know that someone is working on this issue.

Start at the Westend bulkcore1 view linked in the report and trace how it calculates real-world times for sale boundaries. Check whether the stored values are interpreted as local block numbers while the clock now uses relay-chain blocks. Done means the UI shows correct wall-clock estimates for upcoming sale changes on Westend.