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.

native/include/lua/providers.h

@@ -42,6 +42,10 @@ public:
   virtual Status setRotation(int32_t degrees) = 0;
   virtual std::string timezone() const = 0;
   virtual Status setTimezone(const std::string& timezone) = 0;
+  // Only the name of a palette; the colours themselves live in Lua, so the
+  // firmware can read the saved theme before a lua_State exists.
+  virtual std::string theme() const = 0;
+  virtual Status setTheme(const std::string& theme) = 0;
 };
 
 // Only what the firmware alone can answer. App identity, titles, data paths,