Mainnet block-height fallback is hard-coded in script utilities

[Mosas2000]

The helper scripts fall back to static block heights when Hiro API requests fail. That makes market timing increasingly inaccurate over time and can produce bad end/resolution dates whenever the API is unreachable.

Evidence:

• scripts/utils/transaction-helpers.ts returns 6188000 when the mainnet block-height fetch fails.
• scripts/interact-contract.ts returns 6037876 in its own fallback branch.

Impact:

• Deadlines and resolution windows can be calculated from stale chain heights.
• Operators can unknowingly deploy markets with incorrect timing if the API is rate-limited or unavailable.

Reproduction:

1. Disconnect or block access to the Hiro API.
2. Run a script that computes future blocks, such as scripts/bulk-create-markets.ts or scripts/interact-contract.ts.
3. The script silently uses the hard-coded fallback block height instead of a live value.

Priority: High

Suggested scope:
Replace static fallback heights with a safer failure mode or a clearly surfaced manual override, and centralize the fallback policy so all scripts behave consistently.