[Markets] Build multi-token market support in the UI

[samuel2926i39-art]

Summary

Adds TokenSelector plus a formatTokenAmount helper that formats market amounts using the market's actual settlement token symbol and decimal precision, instead of a hardcoded default asset.

Context

• contracts/predict-iq/src/test_multi_token.rs proves markets can be denominated in different tokens; the UI had nothing that read per-market token metadata.
• Before: amount formatting assumed a single default asset (symbol/decimals hardcoded).
• After: amounts are formatted via formatTokenAmount(rawAmount, token), driven by each market's own token symbol/decimals, using BigInt arithmetic so high-decimal tokens (e.g. 18) aren't truncated or rounded by floating point.

Testing

• Manually verified formatting against tokens with 2, 7, and 18 decimals, including trailing-zero trimming and negative amounts.
• Verified low-decimal tokens (e.g. 0 decimals) render whole amounts with no stray decimal point.

Closes #1364

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