Make the AGPL's source offer point at the source being run

Three things a licence audit turned up. None of them is a conflict --
every one of the 182 installed packages is permissive, and the relicence
was within the copyright holder's gift -- but all three are ways the
AGPL fails to stick.

The offer was hard-coded to this repository. Section 13 asks whoever
runs a modified version to offer *that* version's source, so every
deployment with a patch in it was pointing at the wrong tree, and would
have gone on doing so unless its operator noticed and edited the About
page. SOURCE_URL now sets it, alongside APP_NAME, and both the sign-in
page and About read it.

The offer was also only visible after signing in. Whoever is looking at
the sign-in form is interacting with the program over a network too, so
the footer carries it now.

And the two workspace packages declared no licence at all. Private, so
npm never minded, but anything reading the tree saw a blank where the
rest of the project says AGPL-3.0-or-later.

Checked both ways round: with SOURCE_URL set to a fork, the sign-in page
and About both point at the fork; with it unset, both fall back to this
repository.

web/src/views/settings/AboutSettings.tsx

@@ -1,9 +1,12 @@
 import { useSession } from "@/store/session";
 import { client } from "@/jmap/client";
+import { DEFAULT_SOURCE_URL } from "@/lib/source";
 
 export function AboutSettings() {
   const session = useSession((s) => s.session);
   const caps = Object.keys(session?.capabilities ?? {});
+  // A deployment running modified code should offer its own source, not ours.
+  const sourceUrl = session?.ihasmail?.sourceUrl ?? DEFAULT_SOURCE_URL;
   return (
     <div>
       <h1>About ihasmail</h1>
@@ -12,7 +15,7 @@ export function AboutSettings() {
         <img src="/img/logo.png" alt="ihasmail" width={96} />
         <div>
           <div style={{ fontWeight: 700, fontSize: "1.2em" }}>ihasmail 2.0</div>
-          <div className="hint">AGPL-3.0-or-later · <a href="https://github.com/LINUXexpert-org/ihasmail" target="_blank" rel="noreferrer">github.com/LINUXexpert-org/ihasmail</a></div>
+          <div className="hint">AGPL-3.0-or-later · <a href={sourceUrl} target="_blank" rel="noreferrer">{sourceUrl.replace(/^https?:\/\//, "")}</a></div>
         </div>
       </div>
       <h2>Server</h2>