[NePreby]

Summary

Closes #14.

Implements keyboard focus management and trap for Sheet dialogs:

• Focus on open: Automatically saves the triggering element (document.activeElement) and moves keyboard focus into the first focusable element inside the sheet container.
• Tab cycle trap: Traps keyboard navigation within the dialog container using handleFocusTrap (FOCUSABLE_SELECTOR). Pressing Tab wraps from the last interactive element back to the first; pressing Shift+Tab wraps from the first back to the last.
• Focus restoration: Restores focus to the original triggering button/element when the sheet closes via Escape, backdrop click, or close button.
• Unit test suite: Added comprehensive unit tests in src/lib/__tests__/focus.test.ts covering Tab cycle wrap, Shift+Tab cycle wrap, outside focus handling, and non-Tab pass-through. Verified with npx tsc -b and npx vitest run (30/30 passed).

Payout Wallet:

• TRX Tron (TRC20): TMqmJjRgVS21NL2eFtfyHciTxvE4BqGmEv