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/settings.cpp

@@ -10,6 +10,8 @@ extern "C" {
 Settings settings;
 
 static constexpr const char* PATH = "/settings.lua";
+static constexpr const char* TEMP_PATH = "/settings.tmp";
+static constexpr const char* BACKUP_PATH = "/settings.bak";
 
 static int16_t fieldOr(lua_State* L, const char* key, int16_t fallback) {
   lua_getfield(L, -1, key);
@@ -18,6 +20,32 @@ static int16_t fieldOr(lua_State* L, const char* key, int16_t fallback) {
   return value;
 }
 
+static String stringFieldOr(lua_State* L, const char* key, const String& fallback) {
+  lua_getfield(L, -1, key);
+  String value = lua_isstring(L, -1) ? lua_tostring(L, -1) : fallback;
+  lua_pop(L, 1);
+  return value;
+}
+
+static String luaString(const String& value) {
+  String out = "\"";
+  for (size_t i = 0; i < value.length(); i++) {
+    unsigned char c = value[i];
+    if (c == '\\' || c == '\"') {
+      out += '\\';
+      out += (char)c;
+    } else if (c >= 32 && c < 127) {
+      out += (char)c;
+    } else {
+      char escaped[5];
+      snprintf(escaped, sizeof(escaped), "\\%03u", c);
+      out += escaped;
+    }
+  }
+  out += '\"';
+  return out;
+}
+
 bool Settings::setRotation(int16_t degrees) {
   if (degrees % 90 != 0 || degrees < 0 || degrees > 270) return false;
   rotation = degrees;
@@ -44,20 +72,40 @@ bool Settings::load() {
       touchY1 = fieldOr(L, "y1", touchY1);
     }
     lua_pop(L, 1);
+    lua_getfield(L, -1, "wifi");
+    if (lua_istable(L, -1)) {
+      wifiSsid = stringFieldOr(L, "ssid", wifiSsid);
+      wifiPassword = stringFieldOr(L, "password", wifiPassword);
+    }
+    lua_pop(L, 1);
   }
   lua_close(L);
   return ok;
 }
 
 bool Settings::save() const {
-  File f = SD.open(PATH, FILE_WRITE);
+  String source = "return {\n  rotation = " + String(rotation) + ",\n";
+  source += "  touch = { x0 = " + String(touchX0) + ", y0 = " + String(touchY0) +
+            ", x1 = " + String(touchX1) + ", y1 = " + String(touchY1) + " },\n";
+  source += "  wifi = { ssid = " + luaString(wifiSsid) +
+            ", password = " + luaString(wifiPassword) + " },\n}\n";
+
+  if (SD.exists(TEMP_PATH)) SD.remove(TEMP_PATH);
+  File f = SD.open(TEMP_PATH, FILE_WRITE);
   if (!f) return false;
-  char buf[200];
-  int len = snprintf(buf, sizeof(buf),
-                     "return {\n  rotation = %d,\n"
-                     "  touch = { x0 = %d, y0 = %d, x1 = %d, y1 = %d },\n}\n",
-                     rotation, touchX0, touchY0, touchX1, touchY1);
-  bool ok = f.write(reinterpret_cast<const uint8_t*>(buf), len) == (size_t)len;
+  bool ok = f.write(reinterpret_cast<const uint8_t*>(source.c_str()), source.length()) == source.length();
   f.close();
-  return ok;
+  if (!ok) {
+    SD.remove(TEMP_PATH);
+    return false;
+  }
+  if (SD.exists(BACKUP_PATH)) SD.remove(BACKUP_PATH);
+  bool hadSettings = SD.exists(PATH);
+  if (hadSettings && !SD.rename(PATH, BACKUP_PATH)) return false;
+  if (SD.rename(TEMP_PATH, PATH)) {
+    if (hadSettings) SD.remove(BACKUP_PATH);
+    return true;
+  }
+  if (hadSettings) SD.rename(BACKUP_PATH, PATH);
+  return false;
 }