feat(lua): add app navigation history

Make sys.launch push the current path and argument, sys.replace switch without pushing, and sys.back restore the previous route. The status-bar chevron uses the same back action, failed child apps return to their caller, and history is capped at eight routes. Remove the redundant Settings back card.

src/main.cpp

@@ -29,12 +29,38 @@ SPIClass& sdSpi = SPI;
 XPT2046_Touchscreen touch(TOUCH_CS);
 LuaApp app(tft, touch);
 
+struct Route {
+  String path;
+  String arg;
+  bool hasArg = false;
+};
+
+static constexpr size_t MAX_HISTORY = 8;
 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;
+Route currentRoute;
+Route pendingRoute;
+Route history[MAX_HISTORY];
+size_t historyDepth = 0;
+LuaApp::ExitAction pendingAction = LuaApp::ExitAction::None;
+
+Route homeRoute() {
+  Route route;
+  route.path = HOME;
+  return route;
+}
+
+void pushHistory(const Route& route) {
+  if (historyDepth == MAX_HISTORY) {
+    // Home remains the eventual floor; discard the oldest intermediate route.
+    for (size_t i = 1; i < MAX_HISTORY - 1; ++i) history[i] = history[i + 1];
+    historyDepth--;
+  }
+  history[historyDepth++] = route;
+}
+
+Route previousRoute() {
+  return historyDepth > 0 ? history[--historyDepth] : homeRoute();
+}
 
 // Last resort only: the UI lives in Lua, so this exists purely to explain why
 // nothing else could run.
@@ -50,13 +76,15 @@ void fallbackScreen(const char* message) {
   halted = true;
 }
 
-void startApp(const String& path, const char* arg) {
+void startApp(const Route& route) {
   // 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, arg)) return;
-  if (path == HOME) fallbackScreen("home failed to start");
+  Serial.printf("launching %s free=%u largest=%u\n", route.path.c_str(), ESP.getFreeHeap(),
+                ESP.getMaxAllocHeap());
+  const char* arg = route.hasArg ? route.arg.c_str() : nullptr;
+  if (app.load(route.path.c_str(), historyDepth > 0, arg)) return;
+  if (route.path == HOME && historyDepth == 0) fallbackScreen("home failed to start");
 }
 
 void setup() {
@@ -74,7 +102,8 @@ void setup() {
   }
   settings.load();  // absent file keeps the built-in defaults
   net::begin();
-  startApp(HOME, nullptr);
+  currentRoute = homeRoute();
+  startApp(currentRoute);
 }
 
 void loop() {
@@ -88,22 +117,30 @@ void loop() {
   if (app.running()) {
     app.loop();
     if (!app.running()) {
-      nextApp = app.takePendingLaunch();
-      nextArgSet = app.hasPendingArg();
-      nextArg = app.takePendingArg();
+      pendingAction = app.takeExitAction();
+      pendingRoute.path = app.takePendingLaunch();
+      pendingRoute.hasArg = app.hasPendingArg();
+      pendingRoute.arg = 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);
+    // A failed child returns to its caller after leaving the message readable. Home has
+    // no caller, so its failure still reaches the last-resort screen in startApp().
+    if (app.takeFailure()) {
+      delay(ERROR_HOLD_MS);
+      if (pendingAction == LuaApp::ExitAction::None) pendingAction = LuaApp::ExitAction::Back;
+    }
 
-    String path = nextApp.length() > 0 ? nextApp : String(HOME);
-    bool hasArg = nextApp.length() > 0 && nextArgSet;
-    String arg = nextArg;
-    nextApp = "";
-    nextArg = "";
-    nextArgSet = false;
-    startApp(path, hasArg ? arg.c_str() : nullptr);
+    if (pendingAction == LuaApp::ExitAction::Launch) {
+      pushHistory(currentRoute);
+      currentRoute = pendingRoute;
+    } else if (pendingAction == LuaApp::ExitAction::Replace) {
+      currentRoute = pendingRoute;
+    } else {
+      currentRoute = previousRoute();
+    }
+    pendingAction = LuaApp::ExitAction::None;
+    pendingRoute = {};
+    startApp(currentRoute);
   }
 
   // Hands the core to the idle task instead of spinning between polls: touch reads are