feat(lua): pass an argument to init() and let apps name themselves

sys.launch(path, arg) carries a string to the next app's init(arg), nil when there is
none. States share no memory, so one string is the whole handoff; anything structured
travels as a Lua literal the receiver loads. This is what a screen split across apps
needs to say "collect a password for this network".

sys.setAppName() retitles the status bar, defaulting to the directory name as before. A
setter rather than a declared constant, so one app can retitle per screen. The bar needs
no new invalidation path for it -- the name joins rotation and theme in the cache key --
and clips a name wide enough to reach the memory slot.

src/main.cpp

@@ -31,6 +31,10 @@ LuaApp app(tft, touch);
 
 bool halted = false;
 String nextApp;
+// Carried beside the path, because sys.launch()'s argument has to outlive the state that
+// passed it: the sender is torn down before the receiver exists.
+String nextArg;
+bool nextArgSet = false;
 
 // Last resort only: the UI lives in Lua, so this exists purely to explain why
 // nothing else could run.
@@ -46,12 +50,12 @@ void fallbackScreen(const char* message) {
   halted = true;
 }
 
-void startApp(const String& path) {
+void startApp(const String& path, const char* arg) {
   // Full panel until the app's own status bar module reports its height in load().
   tft.setRotation(settings.rotationIndex());  // apps may have rotated the frame
   tft.resetViewport();  // load() re-clips once the new app's bar reports its height
   Serial.printf("launching %s free=%u largest=%u\n", path.c_str(), ESP.getFreeHeap(), ESP.getMaxAllocHeap());
-  if (app.load(path.c_str(), path == HOME)) return;
+  if (app.load(path.c_str(), path == HOME, arg)) return;
   if (path == HOME) fallbackScreen("home failed to start");
 }
 
@@ -70,7 +74,7 @@ void setup() {
   }
   settings.load();  // absent file keeps the built-in defaults
   net::begin();
-  startApp(HOME);
+  startApp(HOME, nullptr);
 }
 
 void loop() {
@@ -83,15 +87,23 @@ void loop() {
 
   if (app.running()) {
     app.loop();
-    if (!app.running()) nextApp = app.takePendingLaunch();
+    if (!app.running()) {
+      nextApp = app.takePendingLaunch();
+      nextArgSet = app.hasPendingArg();
+      nextArg = app.takePendingArg();
+    }
   } else {
     // An app that died left its message on screen, and home is about to paint over it.
     // Serial alone is no help to anyone holding the device.
     if (app.takeFailure()) delay(ERROR_HOLD_MS);
 
     String path = nextApp.length() > 0 ? nextApp : String(HOME);
+    bool hasArg = nextApp.length() > 0 && nextArgSet;
+    String arg = nextArg;
     nextApp = "";
-    startApp(path);
+    nextArg = "";
+    nextArgSet = false;
+    startApp(path, hasArg ? arg.c_str() : nullptr);
   }
 
   // Hands the core to the idle task instead of spinning between polls: touch reads are