fix: replace broken auto-save with explicit Cmd+S save

Removes the debounced auto-save (useAutoSave hook) which was causing the
editor to reload previous content. Replaces it with explicit save on
Cmd+S (⌘S), consistent with the git-based UX of the app.

- Removed useAutoSave hook and its debounce mechanism
- Added useSaveNote hook for direct persist-to-disk
- Added useEditorSave hook that manages pending content buffer + save
- Cmd+S now persists the current editor content immediately
- Rename-before-save: saves pending content before rename to prevent
  the "Failed to rename note" error
- Updated E2E test to verify Cmd+S behavior
- 471 tests passing, code health gates passed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/useSaveNote.ts

@@ -0,0 +1,29 @@
+import { useCallback } from 'react'
+import { invoke } from '@tauri-apps/api/core'
+import { isTauri, mockInvoke, updateMockContent } from '../mock-tauri'
+
+async function persistContent(path: string, content: string): Promise<void> {
+  if (isTauri()) {
+    await invoke('save_note_content', { path, content })
+  } else {
+    await mockInvoke('save_note_content', { path, content })
+  }
+}
+
+/**
+ * Hook that provides an explicit save function for note content.
+ * Called on Cmd+S — no debounce, no auto-save.
+ *
+ * @param updateContent - callback to also update in-memory state after save
+ */
+export function useSaveNote(updateContent: (path: string, content: string) => void) {
+  const saveNote = useCallback(async (path: string, content: string) => {
+    await persistContent(path, content)
+    if (!isTauri()) {
+      updateMockContent(path, content)
+    }
+    updateContent(path, content)
+  }, [updateContent])
+
+  return { saveNote }
+}