fix(handoff): print the shard screen's real path

The end-of-run block told operators to paste the four values at
`<site>/admin/shard`. That page moved when the shard screens became part of the
`uo` module: a module owns one path segment wherever it appears (website
`MODULE_SYSTEM.md` §2.8), so it is `/admin/uo/link`, labelled "Shard (uo-link)".

The old path is worse than a 404. The SPA has no route for it, so it sends the
operator to the dashboard — the link looks like it worked, and the values they
were told to paste have nowhere to go.

- The path is now a named constant, `ADMIN_SHARD_PATH`, carrying why it is not
  the obvious string and the fact that API routes are NOT affected by the module
  namespacing rule (they keep `/api/v1/admin/shard/*`).
- Both handoff tests assert the new path, so this cannot regress quietly.
- The two user-facing labels that name the screen — the `--site-url` help text
  and `update`'s protocol-change instruction — say "Admin → Shard (uo-link)",
  matching what the sidebar actually reads.

Found while writing the runicgateway.com installation journey, by pasting the
printed link into a real deployment and landing on the dashboard.

Co-Authored-By: Claude <noreply@anthropic.com>

src/sidecar.rs

@@ -294,6 +294,18 @@ fn port_of(bind: &str) -> &str {
     }
 }
 
+/// Where the shard settings live in the website's admin panel.
+///
+/// NOT `/admin/shard`, which is what this printed until 2026-08-24 and what an operator who ran
+/// an older build still has in their scrollback. Those screens belong to the `uo` MODULE now, and
+/// a module owns one path segment wherever it appears (website `MODULE_SYSTEM.md` §2.8), so the
+/// page moved. The old path does not 404 — the SPA sends it to the dashboard, which is the worst
+/// way for a link in a handoff to be wrong, because it looks like it worked.
+///
+/// API routes are NOT affected by that rule and keep `/api/v1/admin/shard/*`. This is the SPA URL
+/// a person types.
+const ADMIN_SHARD_PATH: &str = "/admin/uo/link";
+
 /// The end-of-run block from PLAN.md §6 — the one manual step the installer cannot do.
 ///
 /// Returned as a string rather than printed so it can be tested, and so the caller decides where it
@@ -312,12 +324,13 @@ pub fn handoff(doc: &ConfigDoc, host: &str, site_url: Option<&str>) -> String {
          Protocol version  {protocol}\n  \
          Auth token        {token}\n                    \
          (also in {config})\n\n\
-         Paste these into  Admin → Shard  on your Runic Gateway site:\n  \
-         {site}/admin/shard\n\n\
+         Paste these into  Admin → Shard (uo-link)  on your Runic Gateway site:\n  \
+         {site}{admin_path}\n\n\
          The token is write-only once saved — the site will never show it back to you.\n",
         protocol = doc.protocol,
         token = doc.web.auth_token,
         config = doc.config_path,
+        admin_path = ADMIN_SHARD_PATH,
     )
 }
 
@@ -401,7 +414,7 @@ mod tests {
         assert!(block.contains(&doc.web.auth_token), "{block}");
         // The trailing slash on the site URL must not produce a double slash in the link.
         assert!(
-            block.contains("https://my-site.example/admin/shard"),
+            block.contains("https://my-site.example/admin/uo/link"),
             "{block}"
         );
         assert!(block.contains("/etc/runicgateway/sidecar.toml"), "{block}");
@@ -412,7 +425,10 @@ mod tests {
         // An unattended run has nobody to ask, and the token is far too useful to withhold over a
         // link the operator does not need.
         let block = handoff(&doc(), "shard", None);
-        assert!(block.contains("https://<your-site>/admin/shard"), "{block}");
+        assert!(
+            block.contains("https://<your-site>/admin/uo/link"),
+            "{block}"
+        );
         assert!(block.contains("4f9c"), "{block}");
     }