fix(settings): wire autoRefreshInterval, defaultMemo, showTestnetWarning, and slippage (#46)

[ghzhost]

Summary

Addresses and fixes Issue #46: Settings page: autoRefreshInterval, slippageTolerance, defaultMemo, and showTestnetWarning are all saved but never actually applied anywhere — only network works.

Changes Made

1. Extracted useSettings hook (src/hooks/useSettings.ts):
   • Centralized access to stored AppSettings under stellarsend_settings.
   • Also syncs stellarsend_refresh_interval and stellarsend_network for interoperability across tabs/components.
2. autoRefreshInterval:
   • WalletContext.tsx now reads autoRefreshInterval from stellarsend_settings (with fallback to stellarsend_refresh_interval / default 30s).
3. showTestnetWarning:
   • Layout.tsx now gates the testnet banner on both network === 'testnet' and settings.showTestnetWarning.
4. defaultMemo:
   • Send.tsx now seeds SendForm's defaultValues with { memo: settings.defaultMemo } when no form values are yet in progress.
5. slippageTolerance:
   • Added optional slippageTolerance?: string to QuoteRequest in src/types/index.ts.
   • useSendPayment.ts passes settings.slippageTolerance down to quoteApi.getQuote.
6. Tests:
   • Added unit test suite src/hooks/useSettings.test.ts covering initial state, localStorage loading, updates/key syncing, and reset.
   • All 28 test files and 117 tests pass cleanly (vitest).

Fixes #46