Add a presentation mode to the GUI: larger text and lines #21

What is missing

The GUI has one text size. The style sheet sizes text in pixels from a fixed type scale (croak.gui.theme: body 13 px, help 12 px, numbers 19 px), and the plots draw 11 px tick labels with fixed pen widths (croak.gui.plots). There is no larger setting for showing the window on a projector or in a screen share.

Why it matters

Demonstrating a retrieval in a talk, a lab meeting or a video call makes axis labels and the Retrieve numbers too small to read from the back of a room, and the operating system's display scaling enlarges everything, plots' data area included, which leaves the plots cramped at 1366×768.

Context

• The plan for Redesign the GUI as a workflow window (PySide6, pyqtgraph) #16 listed a presentation mode as left for later; the design notes are in docs/developer_guide/gui_design.md.
• The display settings at the foot of the window (croak.gui.display) show how a viewer setting is kept in QSettings and applied to every plot through a signal.

Checked against 5140b0f; read, not run.

To do

• Needs a decision (maintainers): an on/off mode or a size slider. Recommendation: View ▸ Presentation mode, a toggle scaling text and plot lines by about 1.4, remembered between launches.
• A scale factor in theme applied to the type scale when the style sheet is built, and to plot fonts and pen widths, re-applied live without rebuilding the pages.
• Check that every page still fits at 1366×768 in the mode (the existing small-display test, run with it on).
• Tests, and a paragraph in the GUI how-to.

Done when

Toggling the mode changes the text and plot label sizes on every page at once without a restart, the choice survives a relaunch, and the small-display test passes with it on.