feat(config): support *_MB env vars for upload size limits

- Add `applyMegabytesOrBytesEnv` to accept size settings in either bytes or MB
- Prefer `*_BYTES` when set, otherwise convert `*_MB` to bytes with overflow guard
- Add coverage for MB-based environment overrides
- Introduce `static/js/upload-popups.js` to lazy-load and cache popup templatesfeat(config): support *_MB env vars for upload size limits

- Add `applyMegabytesOrBytesEnv` to accept size settings in either bytes or MB
- Prefer `*_BYTES` when set, otherwise convert `*_MB` to bytes with overflow guard
- Add coverage for MB-based environment overrides
- Introduce `static/js/upload-popups.js` to lazy-load and cache popup templates

static/popups/cli.html

@@ -0,0 +1,9 @@
+<h3>Upload with cURL</h3>
+<p>WarpBox accepts normal multipart form uploads through the compatibility endpoint:</p>
+<pre>curl \
+  -F 'files=@./my-file.zip' \
+  -F 'retention=1h' \
+  {{ origin }}/upload
+</pre>
+<h4>Browser flow</h4>
+<p>The browser uses the manifest API: it creates a box, uploads each file, and marks failed uploads so the download page does not wait forever.</p>