fix(gui): render changelog and release notes as Markdown

The changelog dialog and update prompt showed raw Markdown (literal #, **)
instead of rendered styling, making notes hard to read. Render the in-app
changelog with QTextEdit.setMarkdown() and the update prompt's notes as rich
text (Markdown -> HTML via QTextDocument).

Closes #1

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "rigdoctor"
-version = "0.3.0"
+version = "0.3.1"
 description = "Modular hardware monitoring & crash diagnostics for Linux gamers."
 readme = "README.md"
 requires-python = ">=3.11"