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.

.env.example

@@ -2,6 +2,7 @@ WARPBOX_APP_NAME=warpbox.dev
 WARPBOX_ENV=development
 WARPBOX_ADDR=:8080
 WARPBOX_BASE_URL=http://localhost:8080
+WARPBOX_DATA_DIR=./data
 WARPBOX_MAX_UPLOAD_SIZE_MB=2048
 WARPBOX_READ_TIMEOUT=15s
 WARPBOX_WRITE_TIMEOUT=60s