behavior change: right-click context menu now shows "Unbookmark" when
the post is already bookmarked, and "Bookmark as" with a folder submenu
(Unfiled / existing folders / + New Folder) when not. Previously showed
a stateless "Bookmark" action regardless of state.
Step 7 of the gui/app.py + gui/preview.py structural refactor. Pure
move: the embedded preview pane class (the one that lives in the
right column of the main window and combines image+video+toolbar)
is now in its own module. preview_pane.py is at the same package
depth as preview.py was, so no relative-import depth adjustment is
needed inside the class body.
preview.py grows the final preview-side re-export shim line. After
this commit preview.py is just the original imports + _log + shim
block — every class that used to live in it now lives in its
canonical module under media/ or popout/ or as preview_pane. The
file gets deleted entirely in commit 14.
