feat(files-media): group files by directory + add 'Add Directory' button

- render_files_and_media now wraps the per-file loop in directory groups
  via aggregate.group_files_by_dir + imscope.tree_node_ex (mirrors the
  Context Composition visual style at gui_2.py:3114)
- New 'Add Directory' button next to 'Add Files to Inventory':
  uses filedialog.askdirectory() + os.walk to bulk-import a folder tree
- Button IDs (i, add_f_{i}, rem_f_{i}) preserve global uniqueness via
  file_indices map (regression-safe across the directory wrap)
- Test uses mock button=False, mock filedialog.askopenfilenames/askdirectory
  to avoid opening a real Tk dialog during test run

tests/test_files_and_media_tree.py

@@ -0,0 +1,29 @@
+from unittest.mock import patch, MagicMock
+import os, tempfile
+from src import models
+from src.gui_2 import render_files_and_media
+
+def test_files_rendered_under_directory_grouping(app_instance):
+ with tempfile.TemporaryDirectory() as tmp:
+  sub = os.path.join(tmp, "sub")
+  os.makedirs(sub, exist_ok=True)
+  for p in [os.path.join(tmp, "a.py"), os.path.join(tmp, "b.py"), os.path.join(sub, "c.py")]:
+   open(p, "w").close()
+  app_instance.files = [models.FileItem(path=os.path.join(tmp, "a.py")), models.FileItem(path=os.path.join(tmp, "b.py")), models.FileItem(path=os.path.join(sub, "c.py"))]
+  with patch("src.gui_2.imgui") as mock_imgui, patch("src.gui_2.imscope") as mock_imscope, patch("src.gui_2.filedialog") as mock_filedialog, patch("src.gui_2.hide_tk_root", return_value=MagicMock()):
+   mock_imgui.collapsing_header.return_value = True
+   mock_imgui.TableFlags_ = type("T", (), {"resizable": 1, "borders": 2, "row_bg": 4})()
+   mock_imgui.TableColumnFlags_ = type("C", (), {"width_fixed": 1, "width_stretch": 2})()
+   mock_imgui.begin_table.return_value = True
+   mock_imgui.button.return_value = False
+   mock_imscope.group.return_value.__enter__.return_value = None
+   mock_imscope.tree_node_ex.return_value.__enter__.return_value = True
+   mock_filedialog.askopenfilenames.return_value = ()
+   mock_filedialog.askdirectory.return_value = ""
+   try:
+    render_files_and_media(app_instance)
+   except Exception as e:
+    import pytest
+    pytest.fail(f"render_files_and_media raised: {e}")
+  assert len(app_instance.files) == 3
+  assert mock_imscope.tree_node_ex.called, "render_files_and_media should group files under tree_node_ex by directory"