chore(immich): drop the IMMICH_URL hook

A deployment no longer suggests an address for the first server: every
address is each account's own and is typed in the app, so the field opens
blank with a placeholder. The env var, its compose passthrough, the example
line and the `defaultUrl` field that carried it are all gone; the backend's
config route now answers with the saved list alone.

docker/backend/test/immich.mjs

@@ -191,7 +191,6 @@ const server = spawn(process.execPath, entry, {
     PORT: String(PORT),
     DATA_DIR,
     NODE_ENV: 'test',
-    IMMICH_URL: `http://127.0.0.1:${fake.port}`,
   },
   stdio: ['ignore', 'pipe', 'pipe'],
 });
@@ -253,7 +252,7 @@ try {
   check('the key never comes back in clear', !JSON.stringify(saved.body).includes(KEY), JSON.stringify(saved.body));
 
   const config = await user.req('/immich/config');
-  check('the saved list carries the deployment default address', config.body?.defaultUrl === url, JSON.stringify(config.body?.defaultUrl));
+  check('the saved list suggests no address of its own', config.body?.defaultUrl === undefined, JSON.stringify(config.body?.defaultUrl));
   check('the saved server is listed with its version', config.body?.servers?.[0]?.version === '3.1.0', JSON.stringify(config.body?.servers?.[0]));
 
   // A server saved without a name falls back to its host, which is what the tree