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/cli.rs

@@ -77,7 +77,7 @@ pub struct Cli {
     pub patches_unsupported_servuo: bool,
     /// `--host <name>`: the hostname to print in the website URLs.
     pub host: Option<String>,
-    /// `--site-url <url>`: the site's base URL, for the Admin → Shard link.
+    /// `--site-url <url>`: the site's base URL, for the Admin → Shard (uo-link) link.
     pub site_url: Option<String>,
     /// `--yes`: assume the default answer to every prompt.
     pub assume_yes: bool,
@@ -137,7 +137,7 @@ Options:
       --host <NAME>                  install. The hostname to print in the
                                      website URLs.
       --site-url <URL>               install. Your site's base URL, for the
-                                     Admin → Shard link.
+                                     Admin → Shard (uo-link) link.
       --yes                          Assume the default answer to every prompt.
                                      On uninstall it means yes: that prompt
                                      defaults to no, and typing `uninstall