fix: unify settings dropdowns to use custom Radix-based Select

[binaricat]

Summary

Unify all settings dropdown selectors to use the app's custom Radix-based popover design instead of native OS <select> menus.

Problem

The settings page had two inconsistent dropdown styles:

1. Custom styled — Radix-based popovers with app theme (used by FontSelect, TerminalFontSelect)
2. Native OS — Browser <select> with system styling and right-aligned arrow (used by all other settings dropdowns)

Changes

components/settings/settings-ui.tsx

• Replaced native <select> with @radix-ui/react-select in the shared Select component
• Uses portal-based popover content matching FontSelect styling pattern
• Adds check indicator for selected item, chevron icons, scroll buttons
• Same props interface — no changes needed in any consuming components

Affected tabs (no code changes, automatically fixed):

• Appearance — Language selector
• Shortcuts — Hotkey scheme selector
• Terminal — 7 dropdowns (cursor style, scrollback, encoding, etc.)
• System — 1 dropdown