feat: support parallel multi-file uploads

The browser upload flow was built around one selected file and one global
upload state. That made the existing chunk pool useful for a single file, but
users could not start several selected files at the same time.

Refactor the browser state into per-file upload items. Each selected file now
has its own upload record, completed-chunk set, abort controller, retry state,
progress row, and saved IndexedDB resume record. The picker accepts multiple
files, `Start all` and `Resume all` use a bounded file-level pool, and each file
keeps the existing bounded chunk pool. This keeps parallel uploads useful
without letting one large selection create unbounded request fan-out.

Keep the server API unchanged. Each file still receives a separate server upload
id, and server-side progress remains authoritative before any missing chunks are
scheduled. Terminal conflicts still stop the affected file without overwriting
completed data.

Update the user-facing markup, styles, project docs, and test checklist for the
multi-file scheduler. Add a server regression test that interleaves two uploads
and verifies the completed files contain exactly their own bytes.

Test Plan:
- just check
- git diff --check

README.md

@@ -24,7 +24,7 @@ upl
   Browser UI
     static/index.html  upload tool markup
     static/styles.css  responsive tool styling
-    static/app.js      upload scheduler, retries, and browser resume state
+    static/app.js      multi-file scheduler, retries, and browser resume state
   Deployment
     deploy/nginx/      nginx reverse proxy example
     scripts/           reusable local smoke tests
@@ -62,6 +62,13 @@ just run
 
 `just check` also syntax-checks the static browser JavaScript with `node`.
 
+## Browser Uploads
+
+The browser UI accepts multiple selected files. `Start all` runs up to three
+file uploads at the same time, and each file still uploads up to three chunks
+concurrently. Every selected file keeps its own upload id, progress markers,
+abort controller, retry state, and saved IndexedDB resume record.
+
 ## nginx
 
 Run `upl` on localhost and put nginx in front of it for TLS and access control: