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.

README.md

@@ -13,6 +13,9 @@ The default server listens on `:8080`.
 Upload size limits are configured in megabytes through `WARPBOX_MAX_UPLOAD_SIZE_MB`.
 Fractions are supported, so `0.5Mb` is 512 KiB and `1.5Mb` is 1536 KiB.
 
+Runtime data is configured with `WARPBOX_DATA_DIR` and defaults to `./data` in the dev environment.
+The dev script resolves that path from the repository root.
+
 For one-off Go commands, run them from the backend module:
 
 ```bash
@@ -36,6 +39,14 @@ go run ./cmd/warpbox
 - `scripts/env/dev.env.example` - tracked development environment template.
 - `scripts/env/dev.env` - local development environment, ignored by git.
 
+## Runtime Data
+
+Warpbox keeps local runtime data under the configured data directory:
+
+- `data/files/{box_id}/{file_id}.ext` - uploaded file contents.
+- `data/db/warpbox.bbolt` - bbolt metadata database for boxes and file records.
+- `data/logs/{YYYY-MM-DD}.log` - JSONL logs, one event per line.
+
 ## Static Asset Policy
 
-The static handler sets long-lived immutable caching for images, video, audio, and fonts, shorter caching for CSS/JS, and gzip compression for compressible responses. Upload storage is intentionally only a placeholder at this stage.
+The static handler sets long-lived immutable caching for images, video, audio, and fonts, shorter caching for CSS/JS, and gzip compression for compressible responses.