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

lua/lib/ui.lua

@@ -42,7 +42,6 @@ local ui = {}
 ---@field on_cancel? UiHandler
 ---@field on_outside? UiHandler
 
-local THEME_PATH = "/.lua/theme"
 local THEMES = {
   light = { background = { 255, 255, 255 }, color = { 0, 0, 0 }, accent = { 0, 120, 255 }, radius = 6 },
   dark = { background = { 18, 18, 20 }, color = { 235, 235, 235 }, accent = { 166, 118, 255 }, radius = 6 },
@@ -113,7 +112,7 @@ function ui.setTheme(name)
   if not THEMES[name] then
     return nil, "Unknown theme"
   end
-  local ok, err = fs.writeFile(THEME_PATH, name)
+  local ok, err = settings.setTheme(name)
   if not ok then
     return nil, err
   end
@@ -126,8 +125,7 @@ function ui.setTheme(name)
   return true
 end
 
-local savedTheme = fs.readFile(THEME_PATH, 32)
-loadTheme(savedTheme and savedTheme:match "^%s*(.-)%s*$" or "light")
+loadTheme(settings.getTheme())
 
 local function clearState()
   enterHandlers, exitHandlers, clickHandlers, painters = {}, {}, {}, {}

lua/test/ui.lua

@@ -17,6 +17,17 @@ fs = {
   end,
 }
 
+local savedTheme = "light"
+settings = {
+  getTheme = function()
+    return savedTheme
+  end,
+  setTheme = function(name)
+    savedTheme = name
+    return true
+  end,
+}
+
 local frameWidth, frameHeight = 320, 480
 
 gui = {
@@ -166,7 +177,7 @@ assert(table.concat(ui.themeNames(), ",") == "dark,light,mono")
 local ok, err = ui.setTheme "missing"
 assert(ok == nil and err == "Unknown theme")
 assert(ui.setTheme "dark" == true)
-assert(files["/.lua/theme"] == "dark" and ui.getTheme() == "dark")
+assert(savedTheme == "dark" and ui.getTheme() == "dark")
 
 local events = {}
 local function handler(name)