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/styles.css

@@ -88,16 +88,48 @@ h1 {
   clip: rect(0, 0, 0, 0);
 }
 
-.file-summary {
+.upload-section {
   display: grid;
-  gap: 4px;
-  padding: 14px;
+  gap: 12px;
+}
+
+.upload-list {
+  display: grid;
+  gap: 12px;
+  margin: 0;
+  padding: 0;
+  list-style: none;
+}
+
+.upload-item {
+  display: grid;
+  gap: 12px;
+  padding: 12px;
   border: 1px solid var(--line);
   border-radius: 8px;
 }
 
-.file-summary span {
+.upload-item-header {
+  display: grid;
+  grid-template-columns: minmax(0, 1fr) auto;
+  align-items: start;
+  gap: 12px;
+}
+
+.upload-meta {
+  display: grid;
+  gap: 4px;
+  min-width: 0;
+}
+
+.upload-meta span {
   color: var(--muted);
+  font-size: 0.875rem;
+}
+
+.upload-progress {
+  display: grid;
+  gap: 6px;
 }
 
 .progress-wrap {
@@ -115,7 +147,6 @@ h1 {
 }
 
 .progress-meta {
-  margin-top: -12px;
   color: var(--muted);
   font-size: 0.875rem;
 }
@@ -126,6 +157,13 @@ h1 {
   gap: 10px;
 }
 
+.upload-item-actions {
+  display: flex;
+  flex-wrap: wrap;
+  justify-content: flex-end;
+  gap: 8px;
+}
+
 button {
   min-width: 96px;
   min-height: 40px;
@@ -184,7 +222,7 @@ h2 {
 }
 
 .pending-item strong,
-.file-summary strong {
+.upload-meta strong {
   overflow-wrap: anywhere;
 }
 
@@ -232,4 +270,12 @@ h2 {
   .pending-item {
     grid-template-columns: 1fr;
   }
+
+  .upload-item-header {
+    grid-template-columns: 1fr;
+  }
+
+  .upload-item-actions {
+    justify-content: flex-start;
+  }
 }