style: apply Prettier formatting to TS/JS/JSON/MD files

Add Prettier as a dev dependency with .prettierrc.json config and
.prettierignore. Hook added via mirrors-prettier in pre-commit config.
All TypeScript, JSON, Markdown, and YAML files auto-formatted.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.prettierignore

@@ -0,0 +1,9 @@
+dist/
+node_modules/
+coverage.xml
+htmlcov/
+data/
+*.kicad_*
+python/
+package-lock.json
+.venv/