feat: extract mail from mp/mail — initial libreshop/mail

Source moved verbatim from mp/mail/ on 2026-04-29; mp was the first
concrete adapter consuming the libreshop toolkit. Builds and publishes
git.librete.ch/libreshop/mail on every main / v* push via the standard
.gitea/workflows/build.yml shared across libreshop components.

.gitignore

@@ -0,0 +1,15 @@
+.venv
+# libreshop additions
+.env
+.env.local
+.env.*.local
+node_modules/
+.nuxt/
+.output/
+.cache/
+.parcel-cache/
+dist/
+build/
+coverage/
+logs/
+tmp/