fix(client): align maxSpend fee estimation with backend rounding

[cesarenaldi]

Summary

• calculate integer-exponent platform fee factors with exact ratios
• round platform and builder fee reservations at the final five-decimal collateral boundary
• retain an isolated, non-throwing compatibility fallback for fractional exponents
• keep fee-adjusted amounts scaled through market-order amount computation

Stacking

This draft is stacked on the fixed-point order-math branch so the fee work can be reviewed and evolved independently. It should be retargeted to main after the base PR merges.

Verification

• pnpm lint
• pnpm typecheck
• pnpm test:client
• targeted fixed-point and fee tests