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

static/index.html

@@ -18,33 +18,28 @@
         </div>
 
         <div class="file-picker">
-          <button id="pick-button" type="button">Choose file</button>
-          <input id="file-input" type="file">
+          <button id="pick-button" type="button">Choose files</button>
+          <input id="file-input" type="file" multiple>
         </div>
 
-        <div class="file-summary" id="file-summary" hidden>
-          <strong id="file-name"></strong>
-          <span id="file-size"></span>
-        </div>
-
-        <div class="progress-wrap" aria-label="Upload progress">
-          <div class="progress-bar" id="progress-bar"></div>
-        </div>
-        <div class="progress-meta" id="progress-meta">0 of 0 chunks</div>
-
         <div class="actions">
-          <button id="start-button" type="button" disabled>Start</button>
-          <button id="pause-button" type="button" disabled>Pause</button>
-          <button id="resume-button" type="button" disabled>Resume</button>
+          <button id="start-button" type="button" disabled>Start all</button>
+          <button id="pause-button" type="button" disabled>Pause all</button>
+          <button id="resume-button" type="button" disabled>Resume all</button>
         </div>
 
+        <section class="upload-section" id="upload-section" hidden>
+          <h2>Selected uploads</h2>
+          <ul class="upload-list" id="upload-list"></ul>
+        </section>
+
         <section class="pending-section" id="pending-section" hidden>
           <h2>Saved upload progress</h2>
           <ul class="pending-list" id="pending-list"></ul>
         </section>
 
         <ol class="event-log" id="event-log" aria-live="polite">
-          <li>Choose a file to begin.</li>
+          <li>Choose files to begin.</li>
         </ol>
       </section>
     </main>