Show buy-in amount, fee, gas price, and total deduction in join-table preview

[gboigwe]

Summary

TransactionSimulation already estimated the network fee via simulateTransaction, but the modal never showed the buy-in amount itself, an effective gas price, or the total amount that would actually leave the wallet (buy-in + fee) — exactly what this issue asks for.

• TransactionSimulation.tsx accepts an optional buyInAmount (stroops) prop. When present, it renders a "Buy-in Summary" section above the existing generic simulation details: buy-in amount, network fee, gas price (derived from fee / instructions used, both already returned by the existing simulation), and total deduction.
• useJoinTableSimulation now exposes its internal params (tableId, buyIn) so the render site can read the in-flight buy-in amount without new plumbing.
• Table.tsx and page.tsx pass buyInAmount only to the join-table confirmation modal — the separate in-game action confirmation modal (raise/call/etc.) is untouched, since that's not a buy-in flow.

Issue

Closes #161

Notes on interpreting "current XLM/gas price"

Soroban doesn't have a separate market "gas price" the way some other chains do — fees are computed from simulated resource usage against the network's current fee rates, which simulateTransaction (already used here) reflects automatically. I interpreted "current gas price" as the effective rate the simulation implies (fee ÷ instructions used), shown so the user can see how the fee was derived, rather than adding a new external USD/XLM market-price dependency that nothing in the codebase currently has.

Test plan

• npx vitest run — 123 passed, same 4 pre-existing failures as on main before this change (audio-controls.test.tsx, reconnect-state.test.ts — unrelated to this work, confirmed via git stash before/after comparison)
• Added transaction-simulation-buyin.test.tsx — 4 new tests covering: summary shown/hidden based on buyInAmount presence, correct amount/fee/gas-price/total formatting, and no summary when the simulation itself failed
• npx tsc --noEmit — fixed a real compile error my own change introduced (BigInt literal syntax isn't allowed at this project's ES2017 TS target; switched to BigInt(...) calls, matching the existing convention already used elsewhere in this codebase, e.g. page.tsx's STROOPS_PER_XLM). All remaining type errors are pre-existing and unrelated (confirmed present before this change too): token-selector.test.tsx, page.tsx:668 (tokenChoice typing), Table.tsx (replayEntry/setReplayEntry), reconnect-state.ts (ArrayBuffer typing) — not touched, since they aren't caused by this change.

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