Write the brand in lowercase where people see it

The name is inbuxa, lowercase, like the wordmark; INBUXA reads as an
acronym. The admin and webmail already changed. Here that's everything
the server shows people: the brand macro behind the protocol greetings,
the HTTP and SCIM realms, the startup banner and the calendar and contact
PRODID; the first-party OAuth client descriptions; the legacy-protocol
refusals; the default calendar and address book names and the SMTP
greeting default, in the code and the schema served to the admin
(checksum regenerated); startup and shutdown events; the User-Agent;
the sign-in and RSVP pages; the service units; the OpenAPI realm; the
crate descriptions and the README, where it's set in bold.

Identifiers that are uppercase for their own reasons stay: INBUXA_*
settings, SUBSPACE_INBUXA. So do code comments and the AGPL 5(a) notice
lines.

Tests follow: the IMAP ID name, the default collection names, the PRODID
in the iTIP fixtures and the CalDAV free-busy expectations, and the e2e
legacy-protocol refusals. The webdav, imap and jmap suites pass, so do
the unit tests of every crate touched, and 73 of 75 SMTP tests; of the
other two, antispam fails on main too, and queue_retry is a timing flake
that passes on its own.

crates/common/src/manager/first_party.rs

@@ -105,7 +105,7 @@ pub fn first_party_clients(
     if let Some(url) = admin_url.map(|url| url.trim().trim_end_matches('/')).filter(|url| !url.is_empty()) {
         clients.push(FirstPartyClient {
             client_id: ADMIN_CLIENT_ID.to_string(),
-            description: "INBUXA Admin".to_string(),
+            description: "inbuxa Admin".to_string(),
             redirect_uris: vec![format!("{url}/oauth/callback")],
             secret: None,
         });
@@ -356,7 +356,7 @@ mod tests {
 
     fn web_interface() -> Application {
         Application {
-            description: "INBUXA Web Interface".to_string(),
+            description: "inbuxa Web Interface".to_string(),
             enabled: true,
             url_prefix: Map::new(vec!["/admin".into(), "/account".into()]),
             ..Default::default()
@@ -370,7 +370,7 @@ mod tests {
             clients,
             vec![FirstPartyClient {
                 client_id: WEB_INTERFACE_CLIENT_ID.to_string(),
-                description: "INBUXA Web Interface (served by this server)".to_string(),
+                description: "inbuxa Web Interface (served by this server)".to_string(),
                 redirect_uris: vec![
                     "https://mail.example.org/admin/oauth/callback".to_string(),
                     "https://mail.example.org/account/oauth/callback".to_string(),