refactor(wallet): migrate to strict parseTokenAmount + drop dup currency util (S2, #611)

[KruGoL]

What

Migrates the Sphere wallet to the strict amount parser shipped in sphere-sdk@0.10.3 (parseTokenAmount / safeParseTokenAmount), and removes a duplicate currency util. Part of #611 — track S2.

sphere-sdk@0.10.3 removed the lenient toSmallestUnit (silent 0n / truncation). This PR bumps the SDK pin and migrates every call site.

Changes

• Pin @unicitylabs/sphere-sdk 0.10.2 → 0.10.3.
• Submit / build-amount → parseTokenAmount (throws on invalid; caught by the existing try/catch as a clean error):
  • SendModal (handleSend), SendPaymentRequestModal (handleSend), SwapModal (handleSwap), useTopUp.
• Live / keystroke / render → safeParseTokenAmount (null = not a valid amount → button disabled / no proceed; no crash, no silent 0n):
  • SendModal (handleDetailsNext validation + the render-time insufficient-balance check), SendPaymentRequestModal (validation).
• Deleted src/components/wallet/L3/utils/currency.ts — a dead duplicate of the SDK helpers (CurrencyUtils, unused) plus a lossy parseFloat/Math.pow AmountFormatUtils.formatDisplayAmount (its single consumer, PaymentRequestModal, now uses the SDK's bigint-safe formatAmount).

Verification

• tsc -b → clean (exit 0). It caught one real bug during migration — useTopUp passed a number to the now string-only parser; fixed with String(human).
• eslint → clean.

Notes

• sphere-extension is out of scope (deprecated; keeps its own currency copy).
• Independent of the A-track PRs (fix(connect): harden intent handling — validate params, implement receive, scope DM auto-approve (A1-A3, #609) #381/fix(connect): seed approval permissions in an effect, not during render (A4, #609) #382) — different files.