feat: scaffold Firefox extension skeleton with manifest validation tests

Manifest V3 (Firefox keys: background.scripts, host_permissions), a
background event page that opens a standalone list page on toolbar
click, and the placeholder list page (HTML/CSS/JS vanilla). No build
step. extension.test.ts validates manifest shape and referenced files.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/icons/icon.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 96 96" width="96" height="96" role="img" aria-label="Écho du Huit">
+  <rect width="96" height="96" rx="20" fill="#1d4ed8" />
+  <text x="48" y="50" font-family="system-ui, sans-serif" font-size="60" font-weight="700"
+        fill="#ffffff" text-anchor="middle" dominant-baseline="central">8</text>
+</svg>