feat(settings): persist the theme through the settings binding

The palette lives in Lua, so C++ stores the name only and ui.setTheme()
writes through here before rebuilding and repainting.

lua/api/core/settings.lua

@@ -24,3 +24,15 @@ function settings.getTimezone() end
 ---@return true? ok
 ---@return string? error
 function settings.setTimezone(timezone) end
+
+---Returns the saved palette name. Apps read ui.getTheme() instead; this
+---is the stored value, which only ui.setTheme() knows how to apply.
+---@return string
+function settings.getTheme() end
+
+---Persists a palette name without applying it. Call ui.setTheme(), which
+---writes through here and then rebuilds the palette and repaints.
+---@param theme string
+---@return true? ok
+---@return string? error
+function settings.setTheme(theme) end