Prototype: package scaffold and split window

[GeoffRiley]

Parent checkpoint: #2.

Create a minimal installable Python package and one main window with a resizable splitter: editable plain-text pane on the left, image pane on the right. Add a command to open an image and a .md or .txt file manually; no directory scanning yet.

Acceptance

• A fresh checkout can launch the window using documented development commands on Linux.
• Both panes resize and the selected filenames are visible.
• Opening the two sample pairs by hand requires no conversion or import.
• A small smoke test or documented manual check covers startup.

Keep this change focused on structure and opening files. Image interaction and saving can follow separately.