add an alerts module for lua, and implement a volume indicator with it

lua/main.lua

@@ -1,4 +1,36 @@
-require("font")
+local font = require("font")
+local vol = require("volume")
+
+-- Set up property bindings that are used across every screen.
+GLOBAL_BINDINGS = {
+  vol.current_pct:bind(function(pct)
+    require("alerts").show(function()
+      local container = lvgl.Object(nil, {
+        w = lvgl.PCT(80),
+        h = lvgl.SIZE_CONTENT,
+        flex = {
+          flex_direction = "column",
+          justify_content = "center",
+          align_items = "center",
+          align_content = "center",
+        },
+        bg_opa = lvgl.OPA(100),
+        bg_color = "#fafafa",
+      })
+      container:Label {
+        text = string.format("Volume %i%%", pct),
+        text_font = font.fusion_10
+      }
+      container:Bar {
+        w = lvgl.PCT(100),
+        h = 8,
+        range = { min = 0, max = 100 },
+        value = pct,
+      }
+      container:center()
+    end)
+  end),
+}
 
 local backstack = require("backstack")
 local main_menu = require("main_menu")