Convert the visibility toggle to a real element so it can be focused and activated via keyboard (Tab, Enter, Space), and add an aria-label/aria-pressed reflecting hidden/shown state for screen readers.

What does this PR do?

The balance hide/reveal toggle (used across wallet balances, send amounts, orderbook summaries, sweep schedules) was implemented as a plain <span onClick=...> with no keyboard support and no accessible semantics — keyboard-only and screen-reader users could not use it at all.

This PR:

• Changes the clickable wrapper in ElementWithSymbols (Balance.tsx) to render a real <button type="button"> when interactive, instead of a bare <span> (falls back to <span> when the toggle is disabled, preserving existing non-interactive behavior).

• Adds aria-label (reusing the existing settings.show_balance/settings.hide_balance i18n keys) and aria-pressed reflecting the current visibility state.

• Native <button> semantics give keyboard focus and Enter/Space activation for free — no manual tabIndex/onKeyDown needed.

• Adds 5 new tests covering accessible name, aria-pressed state changes, keyboard activation via Enter and Space, and confirming no button is rendered when the toggle is disabled.

• Fixes Balance hide/reveal toggle is not keyboard-accessible or screen-reader-friendly #1409

Visual Demo (For contributors especially)

No visual change. The toggle looks and behaves identically for mouse users. The only difference is it's now also reachable and operable via keyboard (Tab to focus, Enter/Space to activate) and properly announced by screen readers.

Verified via automated tests: npx vitest run --project unit (808/808 relevant tests pass, including 5 new ones), tsc -b build clean, ESLint clean.