feat(wallet): add CSV/JSON export

[Francis6-git]

Description:
Adds client-side CSV and JSON export functionality for the wallet view. Users can export either the entire currently filtered wallet view or selected bulk items without triggering a server round-trip.

Changes Included:

1. src/lib/exportWallet.ts:

• Implemented csvEscape to handle RFC 4180 quoting and tab-prefix formula injection defense (=, +, -, @).
• Added walletItemsToCsv and walletItemsToJson formatting utilities.
• Added triggerDownload, downloadWalletCsv, and downloadWalletJson helpers using temporary Blob object URLs.

2. src/components/wallet/WalletBulkToolbar.tsx:

• Integrated CSV and JSON export triggers.
• Added keyboard navigation controls (ArrowKeys, Home, End, Escape) and ARIA labels.

3. src/lib/tests/exportWallet.test.ts:

• Comprehensive test suite covering formula injection defense, special character escaping, header-only empty views, filtered list handling, and mock DOM download triggers.

Closes #850

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