fix: omit the base path prefix when it resolves to the root

A caller passing basePath '/' produced //api/email/..., which a browser reads as a protocol-relative URL rather than a path. Strip the slashes first and only add a prefix when something is left.

.changeset/fix-cid-attachment-url.md

@@ -7,4 +7,5 @@ Fix the URL that `replaceCidReferences` rewrites inline (CID) attachments to. It
 built `/email/:id/attachment/:file`, which matches no route: the attachment is
 served from `/api/email/:id/attachment/:file`, under the configured base path.
 The URL is now root-relative and includes both parts, so it resolves at the root
-and under a subpath such as `/mail`.
+and under a subpath such as `/mail`. A base path that resolves to the root (`/`)
+adds no prefix, so it can no longer produce a protocol-relative `//api/email/...`.