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.

test/fake_device.lua

@@ -26,8 +26,8 @@ local device = {
   freeHeap = 200000,
   totalHeap = 320000,
   connected = nil,    -- last wifi.connect()
-  exited = false,
-  launched = nil,      -- last sys.launch(), {path, arg}
+  backed = false,
+  launched = nil,      -- last sys.launch()/replace(), {path, arg, replace}
   saveFails = false,  -- make every persisting call report failure
 }
 
@@ -62,7 +62,7 @@ function device.install()
 
   sys = {
     getMillis = function() return device.now end,
-    exit = function() device.exited = true end,
+    back = function() device.backed = true end,
     getAppName = function() return device.appName end,
     setAppName = function(name) device.appName = name end,
     getMemory = function() return device.freeHeap, device.totalHeap end,
@@ -71,7 +71,8 @@ function device.install()
       assert(ms == 0 or on_tick, "setTickInterval without on_tick")
       device.tickInterval = ms
     end,
-    launch = function(path, arg) device.launched = {path = path, arg = arg} end,
+    launch = function(path, arg) device.launched = {path = path, arg = arg, replace = false} end,
+    replace = function(path, arg) device.launched = {path = path, arg = arg, replace = true} end,
   }
 
   settings = {