docs(website): record the uo-link client's config-decrypt contract and the dashboard's mixed tier

Two corrections found by a live smoke test of all 200 routes at every access
level (website PR: fix/uolink-client-throw-and-sitemode-gate).

ARCHITECTURE.md: the "uoLinkClient never throws" invariant was true of the HTTP
call but not of resolving the config, which decrypts the stored auth token and
throws when the ciphertext can't be authenticated (SECRET_ENC_KEY rotated, or a
DB dump restored under a different key). Spell out that this is now handled
inside the client, reported as { ok: false, error: 'uo-link config unreadable' }
with a distinct ERROR log, and that GET /admin/uo-link/config keeps working —
it is the screen an admin needs to re-enter the token and recover.

BACKEND_DESIGN.md: GET /dashboard is staff-wide while PUT /site-mode on the
same screen is adminOnly — the one place a single screen spans two tiers. Note
that the client must gate that control itself rather than relying on the route
gate that admitted the user to the page.

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

website/BACKEND_DESIGN.md

@@ -561,7 +561,13 @@ Public content GETs pass through the **siteMode** gate (§5).
 the whole gate. The ops/config capabilities — `uo-link`, `email`, `discord-bot`, `settings`, and
 `PUT /site-mode` — are `adminOnly`; `shard` is the one mixed prefix, where self-service account
 linking carries no extra gate and the in-game staff operations carry `modAccess`. There is no residual
-file: every admin route is declared in a capability router. The URLs below are unaffected by which
+file: every admin route is declared in a capability router.
+
+`GET /dashboard` and `PUT /site-mode` are the one place where a **single screen spans two tiers**: the
+dashboard is staff-wide, but the site-mode toggle on it is `adminOnly`. The client must therefore gate
+that control on its own (`Dashboard.jsx` renders it only for `role === 'admin'`) rather than relying on
+the route gate that admitted them to the page — the same rule the sidebar follows, so a non-admin is
+never shown a control that would 403. The URLs below are unaffected by which
 file a route sits in — that is the property the route manifest freezes.
 | Method | Path | Purpose |
 |---|---|---|