Fix remaining blocking I/O calls and N+1 project query

- tab_batch_ng.py: async create_batch with to_thread save/sync
- tab_raw_ng.py: async do_save with to_thread, replace deepcopy
  with dict comprehension for display data
- main.py: async create_new with to_thread save
- tab_projects_ng.py: replace per-project count_data_files with
  single list_projects_with_file_counts JOIN query
- db.py: add list_projects_with_file_counts method

Zero blocking I/O calls remain in UI callbacks.

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

main.py

@@ -476,7 +476,7 @@ def render_sidebar(state: AppState, dual_pane: dict):
             with ui.expansion('Create New JSON'):
                 new_fn_input = ui.input('Filename', placeholder='my_prompt_vace').classes('w-full')
 
-                def create_new():
+                async def create_new():
                     fn = new_fn_input.value
                     if not fn:
                         return
@@ -485,7 +485,7 @@ def render_sidebar(state: AppState, dual_pane: dict):
                     path = state.current_dir / fn
                     first_item = copy.deepcopy(DEFAULTS)
                     first_item[KEY_SEQUENCE_NUMBER] = 1
-                    save_json(path, {KEY_BATCH_DATA: [first_item]})
+                    await asyncio.to_thread(save_json, path, {KEY_BATCH_DATA: [first_item]})
                     new_fn_input.set_value('')
                     render_file_list.refresh()