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>

package.json

@@ -61,6 +61,7 @@
         "@jupyterlab/notebook": "^4.0.0",
         "@jupyterlab/services": "^7.0.0",
         "@jupyterlab/settingregistry": "^4.0.0",
+        "@jupyterlab/ui-components": "^4.0.0",
         "@lumino/signaling": "^2.0.0",
         "@lumino/widgets": "^2.0.0",
         "diff": "^5.2.0",