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/update.rs

@@ -81,7 +81,7 @@ pub fn closing(prior: Option<&InstallRecord>, bundle: &Bundle, now: &InstallReco
         println!();
         ui::warn(&format!(
             "The protocol version changed: {} → {}.\n  \
-             Update the Protocol version field in Admin → Shard on your website. Nothing else \
+             Update the Protocol version field in Admin → Shard (uo-link) on your website. Nothing else \
              changed —\n  the URLs and the auth token are the same, and the sidecar answers a \
              website still set to\n  {} with 409 rather than mis-parsing it.",
             previous_protocol.unwrap_or(bundle.protocol),