feat: prefill the mairie form from the picked event

Wire the seam end to end: the background mirrors config.local.json into
storage (a content script would need web_accessible_resources, which would
expose the email to the mairie page), and the content script lays the computed
values into the form.

The layer is deliberately thin and untested: all the logic lives in
formulaire-valeurs.js. It never writes an empty value, never touches the image,
the honeypots or the CSRF tokens, and never submits: a human reads and sends.
A banner states what is left to do, and warns when a field could not be filled,
so that a silent failure stays diagnosable.

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

extension/formulaire-mairie.css

@@ -0,0 +1,49 @@
+/* Bandeau injecté dans la page de la mairie. Propriétés posées explicitement :
+   le CSS du site est hors de notre contrôle et pourrait tout hériter. */
+
+.echo-bandeau {
+  display: block;
+  box-sizing: border-box;
+  margin: 1.5rem 0;
+  padding: 1rem 1.25rem;
+  border: 1px solid #1a73e8;
+  border-left-width: 4px;
+  border-radius: 4px;
+  background: #f2f7fe;
+  color: #1a1a1a;
+  font-family: system-ui, sans-serif;
+  font-size: 1rem;
+  line-height: 1.5;
+  text-align: left;
+}
+
+.echo-bandeau p {
+  margin: 0 0 0.5rem;
+  color: inherit;
+}
+
+.echo-bandeau-titre {
+  font-weight: 700;
+}
+
+.echo-bandeau-liste {
+  margin: 0 0 0.5rem;
+  padding-left: 1.5rem;
+  list-style: disc;
+}
+
+.echo-bandeau-liste li {
+  margin: 0;
+  color: inherit;
+  list-style: disc;
+}
+
+.echo-bandeau-consigne {
+  margin-bottom: 0;
+  font-weight: 700;
+}
+
+.echo-bandeau-alerte {
+  margin: 0.75rem 0 0;
+  color: #b3261e;
+}