feat(wifi): scan, connect and persist networks from the settings app

Adds a wifi binding over the Arduino API and a settings flow that scans, picks
the strongest AP per SSID, takes a password from an on-screen keyboard, and
reports connection state. Credentials join /settings.lua and reconnect at boot.

Settings are now written through a temp file and rename, and strings are
Lua-escaped, so a password cannot corrupt the file the firmware parses at boot.

src/main.cpp

@@ -1,6 +1,7 @@
 #include <SD.h>
 #include <SPI.h>
 #include <TFT_eSPI.h>
+#include <WiFi.h>
 #include <XPT2046_Touchscreen.h>
 
 #include "lua/lua_app.h"
@@ -58,6 +59,11 @@ void setup() {
     return;
   }
   settings.load();  // absent file keeps the built-in defaults
+  WiFi.persistent(false);
+  if (settings.wifiSsid.length()) {
+    WiFi.mode(WIFI_STA);
+    WiFi.begin(settings.wifiSsid.c_str(), settings.wifiPassword.c_str());
+  }
   startApp(LAUNCHER);
 }