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>

yarn.lock

@@ -2785,7 +2785,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"@jupyterlab/ui-components@npm:^4.6.1":
+"@jupyterlab/ui-components@npm:^4.0.0, @jupyterlab/ui-components@npm:^4.6.1":
   version: 4.6.1
   resolution: "@jupyterlab/ui-components@npm:4.6.1"
   dependencies:
@@ -9222,6 +9222,7 @@ __metadata:
     "@jupyterlab/settingregistry": ^4.0.0
     "@jupyterlab/testing": ^4.6.1
     "@jupyterlab/testutils": ^4.0.0
+    "@jupyterlab/ui-components": ^4.0.0
     "@lumino/signaling": ^2.0.0
     "@lumino/widgets": ^2.0.0
     "@module-federation/runtime-tools": ^2.0.0