fix: surface hash-duplicate warning and add manual refresh

Three issues fixed:

1. **Skipped-commit warning not visible** — When the user commits
   unchanged content (hash matches most recent version), the backend
   correctly returns {skipped: true, reason: "unchanged", message: ...}
   but the frontend Notification.warning was easy to miss (default
   4-5s autoClose, small toast at top-right). Now uses {autoClose: false}
   so the warning stays until manually dismissed, plus a console.log
   of the raw commit response for debugging. Added a fallback branch
   that warns clearly if the response shape is unexpected.

2. **No manual refresh on the panel** — In S3 setups the list call
   can occasionally lag the just-committed entry (manifest strong
   consistency is a per-implementation property). Added a "刷新"
   button at the top of the sidebar that calls model.refresh(), letting
   the user force a re-fetch when the auto-refresh misses.

3. **S3 addressing_style** — user-side config addition: S3Storage
   now uses path-style addressing (endpoint/bucket/key) which is the
   format required by MinIO and several other S3-compatible services.

Co-Authored-By: Claude <noreply@anthropic.com>

style/base.css

@@ -13,6 +13,37 @@
   box-sizing: border-box;
 }
 
+.jp-snapshot-panel-container {
+  display: flex;
+  flex-direction: column;
+  height: 100%;
+}
+
+.jp-snapshot-panel-toolbar {
+  display: flex;
+  justify-content: flex-end;
+  margin-bottom: 6px;
+}
+
+.jp-snapshot-panel-refresh-button {
+  font-size: 0.8em;
+  padding: 2px 8px;
+  border: 1px solid var(--jp-border-color2, #ccc);
+  border-radius: 3px;
+  background: var(--jp-layout-color2, #f5f5f5);
+  color: var(--jp-ui-font-color1, #333);
+  cursor: pointer;
+}
+
+.jp-snapshot-panel-refresh-button:hover:not(:disabled) {
+  background: var(--jp-layout-color3, #e8e8e8);
+}
+
+.jp-snapshot-panel-refresh-button:disabled {
+  opacity: 0.6;
+  cursor: not-allowed;
+}
+
 .jp-snapshot-panel-empty {
   color: var(--jp-ui-font-color2, #555);
   padding: 12px;