Refactoring: move all code files into a src directory

src/UI/Base/BackButton.svelte

@@ -0,0 +1,21 @@
+<script lang="ts">
+  /**
+   * Wrapper around 'subtleButton' with an arrow pointing to the right
+   * See also: NextButton
+   */
+  import SubtleButton from "./SubtleButton.svelte"
+  import { ChevronLeftIcon } from "@rgossiaux/svelte-heroicons/solid"
+  import { createEventDispatcher } from "svelte"
+  import { twMerge } from "tailwind-merge"
+
+  const dispatch = createEventDispatcher<{ click }>()
+  export let clss: string | undefined = undefined
+</script>
+
+<SubtleButton
+  on:click={() => dispatch("click")}
+  options={{ extraClasses: twMerge("flex items-center", clss) }}
+>
+  <ChevronLeftIcon class="h-12 w-12" slot="image" />
+  <slot slot="message" />
+</SubtleButton>