feat: initialize warpbox.dev project structure and backend

Initialize the repository with the core Go backend architecture and a frontend mockup for warpbox.dev, a self-hosted file-sharing application.

- Set up Go backend modules for configuration, HTTP server, middleware, handlers, and templates.
- Add local development scripts, environment templates, and basic project configuration.
- Include a React-based frontend mockup under the docs directory.

backend/static/js/app.js

@@ -0,0 +1,42 @@
+(function () {
+  const dropZone = document.querySelector(".drop-zone");
+  const fileInput = document.querySelector("#file-input");
+
+  if (!dropZone || !fileInput) {
+    return;
+  }
+
+  ["dragenter", "dragover"].forEach((eventName) => {
+    dropZone.addEventListener(eventName, (event) => {
+      event.preventDefault();
+      dropZone.classList.add("is-dragging");
+    });
+  });
+
+  ["dragleave", "drop"].forEach((eventName) => {
+    dropZone.addEventListener(eventName, (event) => {
+      event.preventDefault();
+      dropZone.classList.remove("is-dragging");
+    });
+  });
+
+  dropZone.addEventListener("drop", (event) => {
+    if (event.dataTransfer && event.dataTransfer.files.length > 0) {
+      fileInput.files = event.dataTransfer.files;
+      updateDropLabel(event.dataTransfer.files.length);
+    }
+  });
+
+  fileInput.addEventListener("change", () => {
+    updateDropLabel(fileInput.files.length);
+  });
+
+  function updateDropLabel(count) {
+    const title = dropZone.querySelector(".drop-title");
+    if (!title) {
+      return;
+    }
+
+    title.textContent = count === 1 ? "1 file selected" : `${count} files selected`;
+  }
+})();