ui: preview/edit toggle for markdown, empty-pane open cards, + flush to the rightmost tab

- markdown tabs open in rendered preview with a 预览/编辑 segmented toggle;
  edit mode uses the CodeMirror source editor (markdown highlighting), the
  preview renders the live unsaved draft, and the editor stays mounted while
  previewing so edits survive the toggle.
- an empty pane shows openable-type cards (explorer / git / terminal, the
  same options as the + menu, terminal dimmed at the cap) instead of the
  bare 'this pane is empty' message; clicking a card opens the tab.
- the + button sits immediately right of the last tab (sticky at the
  scrollport's right edge when the tab row overflows, so it stays
  reachable).

tests/unit.spec.ts

@@ -310,6 +310,7 @@ describe('editor language mapping', () => {
     expect(languageKeyForExt('py')).toBe('python')
     expect(languageKeyForExt('yaml')).toBe('yaml')
     expect(languageKeyForExt('sh')).toBe('shell')
+    expect(languageKeyForExt('md')).toBe('md')
     expect(languageKeyForExt('txt')).toBeNull()
     expect(languageKeyForExt('log')).toBeNull()
     expect(languageKeyForExt('')).toBeNull()