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>

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to RigDoctor are recorded here. Format follows
 (`MAJOR.MINOR.PATCH`, pre-1.0). `__version__` and `pyproject.toml` must match the git
 release tag (so the auto-updater, D18, can compare versions).
 
+## [0.3.1] - 2026-05-21
+### Fixed
+- Changelog/release notes now **render Markdown** instead of showing raw `#`/`**` markup —
+  the in-app changelog uses `QTextEdit.setMarkdown()` and the update prompt renders notes as
+  rich text (closes #1).
+
 ## [0.3.0] - 2026-05-21
 ### Added
 - **System inventory (M5)**: CPU, GPU (model/driver/VBIOS/VRAM/PCIe), motherboard/BIOS, RAM