sidebar: VSCode-style drag-to-edge splitting, collapse instead of close, theme-compliant tokens

- Split panes by dragging a tab to a pane edge (left/right/up/down zones
  with a drop overlay) or into the center to merge; the toolbar split
  buttons are gone (split-pane.tsx, state.ts moveTabToEdge/insertLeafAt).
- The header close button is now a collapse button; the expand button on
  the right edge stays visible whenever the panel is collapsed, so the
  sidebar can always be reopened (Sidebar.tsx, locales collapse/expand).
- Styles now follow the DSH web-styling rules exactly: --dsw-alias-*
  semantic colors (state-error/warn tokens), --dsw-font-* typography roles
  (size + line-height paired), --dsw-shadow-lv* elevation, no color
  literals or static palette values (sidebar.module.css).
- Tests: +3 for moveTabToEdge (edge split, center merge, self reorder).

src/client/locales.ts

@@ -16,6 +16,8 @@ const zh = {
   newTerminal: '新终端',
   terminalLimit: '终端数量已达上限 (3)',
   close: '关闭',
+  collapse: '折叠侧边栏',
+  expand: '展开侧边栏',
   refresh: '刷新',
   save: '保存',
   saved: '已保存',
@@ -71,6 +73,8 @@ const en: Record<keyof typeof zh, string> = {
   newTerminal: 'New terminal',
   terminalLimit: 'Terminal limit reached (3)',
   close: 'Close',
+  collapse: 'Collapse sidebar',
+  expand: 'Expand sidebar',
   refresh: 'Refresh',
   save: 'Save',
   saved: 'Saved',