fix(bar): use overlay layer for hide and overlay modes

This fixes a major inconsistency with the swaybar implementation of
these modes[^1]. `overlay` layer no longer has security implications due
to a wide adoption of `ext-session-lock`, so it's safe to use.

Following config will restore the previous behavior:
```json
"modes": {
    "hide": { "layer": "top" },
    "overlay": { "layer": "top" }
},
```

[^1]: 2f7247e08a

src/bar.cpp

@@ -37,7 +37,7 @@ const Bar::bar_mode_map Bar::PRESET_MODES = {  //
       .visible = true}},
     {"hide",
      {//
-      .layer = bar_layer::TOP,
+      .layer = bar_layer::OVERLAY,
       .exclusive = false,
       .passthrough = false,
       .visible = true}},
@@ -49,7 +49,7 @@ const Bar::bar_mode_map Bar::PRESET_MODES = {  //
       .visible = false}},
     {"overlay",
      {//
-      .layer = bar_layer::TOP,
+      .layer = bar_layer::OVERLAY,
       .exclusive = false,
       .passthrough = true,
       .visible = true}}};