docs: use a dark-themed README screenshot

The previous capture rendered light because the offscreen Qt platform ships a
light default palette, which is not how the application looks on the maintainer's
desktop. The capture script now applies a Fusion dark palette before constructing
the window.

Verified by sampling the rendered PNG rather than by eye: every background region
measures dark — results viewport and history, catalog and schema cells at
#1e1e1e, alternating rows at #2d2d2d, menu and status bars at #353535. An earlier
read of "still light" came from sampling text glyphs instead of background.

cacheBuster incremented so the new image is served rather than the cached one.

Author: beallio

README.md

@@ -1,16 +1,16 @@
 # Wherewolf
 
-<img src="https://raw.githubusercontent.com/beallio/wherewolf/main/src/wherewolf/assets/img/wherewolf_banner.png?cacheBuster=22" width="100%">
+<img src="https://raw.githubusercontent.com/beallio/wherewolf/main/src/wherewolf/assets/img/wherewolf_banner.png?cacheBuster=23" width="100%">
 
-[![CI](https://github.com/beallio/wherewolf/actions/workflows/ci.yml/badge.svg?cacheBuster=22)](https://github.com/beallio/wherewolf/actions/workflows/ci.yml)
-[![PyPI version](https://img.shields.io/pypi/v/wherewolf.svg?cacheBuster=22)](https://pypi.org/project/wherewolf/)
-[![License: GPL-3.0-only](https://img.shields.io/badge/License-GPL--3.0--only-blue.svg?cacheBuster=22)](https://www.gnu.org/licenses/gpl-3.0.html)
+[![CI](https://github.com/beallio/wherewolf/actions/workflows/ci.yml/badge.svg?cacheBuster=23)](https://github.com/beallio/wherewolf/actions/workflows/ci.yml)
+[![PyPI version](https://img.shields.io/pypi/v/wherewolf.svg?cacheBuster=23)](https://pypi.org/project/wherewolf/)
+[![License: GPL-3.0-only](https://img.shields.io/badge/License-GPL--3.0--only-blue.svg?cacheBuster=23)](https://www.gnu.org/licenses/gpl-3.0.html)
 
 Wherewolf is a local SQL workbench for CSV, Parquet, JSON, JSON Lines, and XLSX files. It opens
 a native PyQt6 desktop window and runs queries with DuckDB by default. There is no browser UI and
 no local web server.
 
-![Wherewolf Screenshot](https://raw.githubusercontent.com/beallio/wherewolf/main/src/wherewolf/assets/img/screenshot.png?cacheBuster=22)
+![Wherewolf Screenshot](https://raw.githubusercontent.com/beallio/wherewolf/main/src/wherewolf/assets/img/screenshot.png?cacheBuster=23)
 
 ## Install