ui(settings): fix dropdowns, move hotkey to general, unify bg colors

- select: wider min-width, proper padding, custom chevron
- hotkey: moved from Hotkey page to General > Shortcut group
- renamed 'Toggle Shortcut' to 'Hotkey', removed Hotkey nav/page
- .row bg: #242424 with hover state
- nav + content bg: #1F1F1F (unified)
- --surface: #1F1F1F, --bg-2: #1F1F1F