Add Makefile, package.json, and initial configuration for VitePress and Slidev integration

- Introduced a Makefile for streamlined commands related to building, running, and managing presentations.
- Created package.json to define project metadata, dependencies, and scripts for development and testing.
- Added .gitignore entries to exclude build artifacts and temporary files from version control.
- Included playwright.config.mjs for end-to-end testing setup with Playwright.
- Added README.md for project documentation, outlining setup instructions and development guidelines.

slides/_shared/center-default-like-reveal.css

@@ -0,0 +1,11 @@
+/*
+ * Reveal : center: true — le thème Slidev « default » aligne le bloc en haut-gauche.
+ * Si une planche reste en layout default (cache, ancien bundle, etc.), on centre
+ * le bloc comme le layout intégré « center ». Ne cible pas .slidev-layout.center.
+ */
+.slidev-layout.default {
+  min-height: 100%;
+  display: grid;
+  place-content: center;
+  box-sizing: border-box;
+}