feat: add configurable data directory and file-based logging

Introduce the `WARPBOX_DATA_DIR` environment variable to define where runtime data is stored. This directory will house uploaded files, the bbolt metadata database, and application logs.

Changes include:
- Added `WARPBOX_DATA_DIR` to configuration, defaulting to `./data`.
- Implemented a custom logging package that writes JSONL logs to the data directory.
- Updated `.gitignore` and `.env.example` to support the new data directory.
- Documented the runtime data structure in `README.md`.
- Updated the frontend upload script to handle form submission and display results.

backend/libs/handlers/app.go

@@ -27,8 +27,11 @@ func NewApp(cfg config.Config, logger *slog.Logger, renderer *web.Renderer, uplo
 
 func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /", a.Home)
+	mux.HandleFunc("GET /d/{boxID}", a.DownloadPage)
+	mux.HandleFunc("GET /d/{boxID}/zip", a.DownloadZip)
+	mux.HandleFunc("GET /d/{boxID}/f/{fileID}", a.DownloadFile)
 	mux.HandleFunc("GET /healthz", a.Health)
 	mux.HandleFunc("GET /api/v1/health", a.Health)
-	mux.HandleFunc("POST /api/v1/upload", a.UploadPlaceholder)
+	mux.HandleFunc("POST /api/v1/upload", a.Upload)
 	mux.Handle("GET /static/", a.Static())
 }