feat(vcs): diff preview in the Git window's Local Changes tab (#714)

The Local Changes tab of the Git tool window showed only the changes tree; a file's diff was visible only after opening a diff tab.

It now previews the selected file's working-tree diff beside the tree when the window is docked at the bottom and below it when docked on a side. The preview follows the selection (arrow keys included), shows a placeholder for no or multiple selection, and can be turned off with View Options › Show Diff Preview (persisted; the selection is kept when toggling).

The preview reuses the diff editor, whose toolbar did not fit a side-docked window: the editor is now a size container, the whitespace and highlighting pickers move to the settings menu below 640px and the separators go below 380px, buttons no longer shrink and the differences counter truncates.

Fixes #710.

Co-authored-by: a-rookie-of-C-language <[email protected]>

src/platform/locales/text/zh.json

@@ -1755,6 +1755,7 @@
   "Restore Size": "恢复大小",
   "Restore detected roots": "恢复检测到的根",
   "Result": "结果",
+  "Select a file to preview its changes": "选择文件以预览其更改",
   "Select a problem to see its details": "选择一个问题以查看详情",
   "Select an item to preview": "选择一项以预览",
   "Severity": "严重性",