feat: introduce a shared config with an optional local override

The mairie form needs an organiser, an email and the café defaults. Ship the
defaults in the repo and let each volunteer override the personal bits in
config.local.json (gitignored, optional: a fresh clone still works).

config.js carries neither import nor export on purpose, so that it loads both
as a classic script (content script, background) and as an ESM module. That is
what lets presentation.js drop its duplicated café address.

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

extension/config.local.json.example

@@ -0,0 +1,4 @@
+{
+  "organisateur": "L'Atelier du Huit",
+  "email": "contact@atelier-huit.fr"
+}