The source offer is a link to this fork, not a tarball beside the app (#7)

INBUXA Admin built its own source into every deployment: the whole tree
packed as source.tar.gz next to the app, with an identity string shown
beside the link and in the version tooltip. The top bar menu and the
source link offered that download.

It answered the AGPL precisely -- the source of *this* build, uncommitted
work and all -- but it paid for that by carrying the tree into every
deployment, to a repository that is public and already has it. The
version shown beside the link already names the build, so the link and
the version together say what the archive said.

This is the same change already made to ihasmail-inbuxa, for the same
reason, and it leaves the two forks answering the offer the same way.

It is also what lets INBUXA Admin be built in an Alpine image: the
plugin shelled out to `tar --transform`, which BusyBox tar does not
have, so `npm run build` failed there and nowhere else.

Removed with it: source-archive.ts, the Vite plugin call, __SOURCE_ID__
and sourceDownloadUrl(). The build no longer shells out to git or tar,
and nothing is written next to the app.

The two strings that named the archive change with it: the link now
reads "Source code (<version>), AGPL-3.0" rather than naming a tree id,
and the version tooltip drops the id it appended.

src/lib/sourceDownload.ts

@@ -4,9 +4,11 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
-import { getBasePath } from '@/lib/basePath';
-
-/** Where this build's own source is: written next to the app at build time (see source-archive.ts). */
-export function sourceDownloadUrl(): string {
-  return `${getBasePath()}/source.tar.gz`;
-}
+/**
+ * Where to point someone who wants this interface's source.
+ *
+ * INBUXA Admin is a modified Stalwart web interface, so the AGPL's offer is
+ * this fork. The version shown beside the link names the build, which is what
+ * makes the offer something a person can act on.
+ */
+export const SOURCE_URL = 'https://github.com/inbuxa/inbuxa-admin';