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/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 = {}, {}, {}, {}