refactor: remove tab bar — single note open at a time

Replace the multi-tab model with single-note navigation. One note is
open at a time; clicking any note (sidebar, wikilink, relationship)
replaces the current note in the editor. Back/Forward history still
works via Cmd+[/].

Removed: TabBar component, useClosedTabHistory, tab reorder/close/
reopen logic, Cmd+W/Cmd+Shift+T shortcuts, Close Tab and Reopen
Closed Tab menu items, tabLayout utility, and all related tests.

Simplified: useTabManagement (single-note), useAppNavigation (no tab
lookup), useKeyboardNavigation (note nav only), useNoteCreation (no
close-tab cleanup), useDeleteActions (deselect instead of close tab),
Editor (no TabBar render), Rust menu (removed 2 menu items).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/hooks/useNoteActions.test.ts

@@ -941,30 +941,6 @@ describe('useNoteActions hook', () => {
       expect(removeEntry).not.toHaveBeenCalled()
     })
 
-    it('closing unsaved tab removes entry', () => {
-      const clearUnsaved = vi.fn()
-      const unsavedPaths = new Set<string>()
-      const config = makeConfig()
-      config.clearUnsaved = clearUnsaved
-      config.unsavedPaths = unsavedPaths
-
-      const { result } = renderHook(() => useNoteActions(config))
-
-      act(() => {
-        result.current.handleCreateNoteImmediate()
-      })
-
-      const createdPath = addEntry.mock.calls[0][0].path
-      unsavedPaths.add(createdPath) // simulate trackUnsaved
-      config.unsavedPaths = unsavedPaths // update ref
-
-      act(() => {
-        result.current.handleCloseTab(createdPath)
-      })
-
-      expect(removeEntry).toHaveBeenCalledWith(createdPath)
-      expect(clearUnsaved).toHaveBeenCalledWith(createdPath)
-    })
   })
 
   describe('type change does not move file', () => {