feat: add browser resumable upload client

Replace the placeholder browser script with the PLAN.md upload flow. The static
UI now creates upload records, slices the selected file into fixed-size chunks,
uploads missing chunks with a concurrency pool of three workers, retries failed
chunks with exponential backoff, pauses via AbortController, and completes the
upload once the server has accepted every chunk.

Persist pending upload records in IndexedDB and render them in the page so a
reload can resume from server-authoritative progress. When the File System
Access API is available, the app stores a file handle and asks for read
permission during resume; when it is unavailable or permission is denied, the
same pending record resumes after the user reselects the matching file. Browser
state is helpful but not trusted: every resume starts by querying the server for
completed chunks.

Add a JavaScript syntax check to the justfile, update the static-page test and
documentation, and extend TESTS.md with the manual resume scenarios that still
need real-browser repetition.

Test Plan:
- just check
- UPL_BIND=127.0.0.1:39123 UPL_DATA_DIR=$(mktemp -d) cargo run
- curl -fsS http://127.0.0.1:39123/healthz
- curl -fsS http://127.0.0.1:39123/ | rg "Choose file|Pending uploads|app.js"
- firefox --headless --screenshot /tmp/upl-page.png http://127.0.0.1:39123/

Refs: PLAN.md milestones 5, 6, and 7

TESTS.md

@@ -18,6 +18,7 @@ Keep this file as the reusable verification checklist while implementing
     - `GET /api/uploads/:id` reports completed chunks from disk.
     - `POST /api/uploads/:id/complete` assembles verified chunks.
     - `POST /api/uploads/:id/complete` rejects incomplete uploads.
+    - `static/app.js` passes `node --check`.
 
 ## Manual
 
@@ -29,6 +30,12 @@ features land.
 - Kill the browser tab mid-upload and resume.
 - Restart the Rust server mid-upload and resume.
 - Interrupt the network and resume.
+- Pause from the browser controls and resume.
+- Reload the page and resume from the pending upload list.
+- In a browser with the File System Access API, resume without reselecting the
+  file after granting read permission.
+- In a browser without the File System Access API, resume after reselecting the
+  same file.
 - Retry a duplicate chunk and confirm it is accepted idempotently.
 - Attempt an invalid chunk index and confirm it is rejected.
 - Attempt a wrong-size non-final chunk and confirm it is rejected.