feat: replace text buttons with icons (tooltip on hover)

Replaces the two visible 'button-like' affordances with LabIcon glyphs
so the sidebar and toolbar stay compact; the explanatory text moves
to the hover tooltip.

Changes:
- Toolbar 'Save Snapshot' button: text label -> saveIcon (floppy),
  tooltip '保存当前 notebook 为快照'
- Sidebar panel '快照历史' text label -> saveIcon + caption='快照历史'
  (the panel tab is now icon-only)
- Sidebar refresh button: text -> refreshIcon with a 1s rotate animation
  while loading

Dependencies:
- Adds @jupyterlab/ui-components to package.json (source of the
  standard LabIcon set: saveIcon, refreshIcon, historyIcon)

Style:
- New .jp-snapshot-panel-refresh-button sizing (28x24 fixed box,
  centered icon) and .is-loading spin keyframes
- @keyframes jp-snapshot-spin applied to the icon's .jp-Icon element

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

src/index.ts

@@ -5,6 +5,7 @@ import {
 import { INotebookTracker, NotebookPanel } from '@jupyterlab/notebook';
 import { ToolbarButton, MainAreaWidget } from '@jupyterlab/apputils';
 import { ISettingRegistry } from '@jupyterlab/settingregistry';
+import { saveIcon } from '@jupyterlab/ui-components';
 
 import { requestAPI } from './request';
 import { registerCommands, CommandIDs } from './commands';
@@ -59,7 +60,11 @@ const plugin: JupyterFrontEndPlugin<void> = {
       onOpenDiff: openDiff
     });
     panel.id = 'snapshot:panel';
-    panel.title.label = '快照历史';
+    // Icon-only panel tab: the title text becomes a hover caption so the
+    // sidebar stays compact. saveIcon is reused for visual consistency with
+    // the commit toolbar button (both signal "save" semantics).
+    panel.title.icon = saveIcon;
+    panel.title.caption = '快照历史';
     app.shell.add(panel, 'left', { rank: 200 });
 
     // Keep the model in sync with the current notebook.
@@ -73,13 +78,14 @@ const plugin: JupyterFrontEndPlugin<void> = {
 
     registerCommands(app, tracker, model);
 
-    // Add a "保存快照" button to every notebook panel's toolbar.
+    // Add an icon-only "保存快照" button to every notebook panel's toolbar.
+    // The tooltip on hover carries the full description.
     // NOTE: docRegistry.addWidgetExtension('Notebook', ...) places its
     // returned widget in the *main area* (a new tab), not on the toolbar.
     // The toolbar wants panel.toolbar.addItem directly.
     const addCommitButton = (panelHost: NotebookPanel) => {
       const button = new ToolbarButton({
-        label: '保存快照',
+        icon: saveIcon,
         tooltip: '保存当前 notebook 为快照',
         onClick: () => {
           void app.commands.execute(CommandIDs.commit);