feat: hooks first architecture

[Copilot]

Refactors vault service to a hooks-first architecture, separating business logic (hooks) from pure utility functions (services) and presentation (components).

Architecture Changes

• Three-layer service structure: calculations, validations, and transformers as pure functions
• React hooks orchestration: deposit flow logic moved to useDepositValidation and useDepositTransaction
• Documentation: added architecture guide and migration patterns

Bug Fixes

• Precision safety in formatSatoshisToBtc: replaced Number conversion with bigint arithmetic throughout to prevent loss for values ≥ 2^53
• Input validation in parseBtcToSatoshis: handles edge cases (empty strings, decimals-only input like ".5", multiple decimal points)
• Magic numbers extracted: fee calculation constants (BASE_TX_SIZE, PER_INPUT_SIZE, FEE_RATE) now named and configurable
• Unused imports removed: cleaned up useDepositTransaction and useDepositValidation

// Before: precision loss risk
const btc = Number(satoshis) / 100_000_000;

// After: bigint arithmetic
const whole = satoshis / SATOSHIS_PER_BTC;
const fraction = satoshis % SATOSHIS_PER_BTC;

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