feat: right-click "Duplicate tab" — clone files into a new tab with adapted name + own folder

New tab copies the source tab's video list + separators, gets a unique
"<name> copy" label and an adapted own export folder ("<folder>_copy"), and
inherits the tab-named-folder flag. No files are moved or copied — you export
into the new tab's folder. Keeps Foley/variant datasets separate without the
file-shuffling that a misexport used to require.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_ui_structure.py

@@ -107,3 +107,22 @@ def test_side_by_side_menu_pins_third_panel(win):
         win._deck_loading = False
     assert win._tab_crop._pinned is True
     assert len(_split_columns(win)) == 3
+
+
+def test_duplicate_tab(win):
+    # Right-click → Duplicate tab: clones files into a new tab with an adapted
+    # name + adapted own folder, no file moves. Suppress QSettings writes via
+    # _loading_tabs so the test can't touch the real session.
+    win._loading_tabs = True
+    try:
+        src = win._pws[0]
+        src._label = "AlexisCrystal"
+        src._dest_folder = "/data/alexis"
+        n_before = len(win._pws)
+        win._on_duplicate_tab(win._playlist_tabs.indexOf(src))
+    finally:
+        win._loading_tabs = False
+    assert len(win._pws) == n_before + 1
+    dup = win._pws[-1]
+    assert dup._label == "AlexisCrystal copy"
+    assert dup._dest_folder == "/data/alexis_copy"