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,

native/src/bindings/core/settings.cpp

@@ -47,6 +47,19 @@ int setTimezone(lua_State* state) {
       state, Runtime::from(state)->settings().setTimezone({value, length}));
 }
 
+int getTheme(lua_State* state) {
+  const std::string theme = Runtime::from(state)->settings().theme();
+  lua_pushlstring(state, theme.data(), theme.size());
+  return 1;
+}
+
+int setTheme(lua_State* state) {
+  size_t length = 0;
+  const char* value = luaL_checklstring(state, 1, &length);
+  return pushStatus(state,
+                    Runtime::from(state)->settings().setTheme({value, length}));
+}
+
 const luaL_Reg FUNCTIONS[] = {
     // --- Returns the saved rotation in degrees clockwise.
     // @return integer
@@ -64,6 +77,16 @@ const luaL_Reg FUNCTIONS[] = {
     // @return true|nil ok
     // @return string|nil error
     {"setTimezone", setTimezone},
+    // --- 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
+    {"getTheme", getTheme},
+    // --- 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|nil ok
+    // @return string|nil error
+    {"setTheme", setTheme},
     {nullptr, nullptr},
 };
 

native/test/fake_providers.h

@@ -26,6 +26,7 @@ struct Log : LogProvider {
 struct Settings : SettingsProvider {
   int32_t degrees = 0;
   std::string tz = "UTC0";
+  std::string themeName = "light";
   int32_t rotation() const override { return degrees; }
   Status setRotation(int32_t value) override {
     degrees = value;
@@ -36,6 +37,11 @@ struct Settings : SettingsProvider {
     tz = value;
     return Status::success();
   }
+  std::string theme() const override { return themeName; }
+  Status setTheme(const std::string& value) override {
+    themeName = value;
+    return Status::success();
+  }
 };
 
 struct Sys : SysProvider {