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

TESTS.md

@@ -21,6 +21,8 @@ Keep this file as the reusable verification checklist while implementing
     - `GET /api/uploads/:id` reports completed chunks from disk markers.
     - `POST /api/uploads/:id/complete` renames the verified temp upload file
       and removes staging data.
+    - Parallel upload requests for separate files complete without crossing
+      bytes between temp upload files.
     - `POST /api/uploads/:id/complete` rejects incomplete uploads.
     - `POST /api/uploads/:id/complete` rejects tampered temp upload files.
     - `static/app.js` passes `node --check`.
@@ -38,6 +40,8 @@ deployment retests.
 
 - Upload a small file in one pass.
 - Upload a file larger than one chunk.
+- Select multiple files and confirm several upload rows advance at the same
+  time.
 - Kill the browser tab mid-upload and resume.
 - Restart the Rust server mid-upload and resume.
 - Interrupt the network and resume.