Responsive Layout for Large Monitors

Problem

The UI wastes large amounts of screen space on high-resolution displays.

Acceptance Criteria

* Start maximized (or remember previous size).
* Responsive pane sizing.
* Dynamic minimum widths.
* Better default splitter ratios.
* DPI-aware scaling

No files, tests, or entry points are named; begin by tracing the desktop UI startup and pane or splitter layout code. Done means the acceptance criteria work on large and high-DPI displays, including startup or remembered window size, responsive panes, minimum widths, splitter ratios, and scaling.

Written by the indexing model from the issue text.