fix(web): don't copy a cloned server's other settings (#1915)

Review follow-ups on #2038.

`configModalTarget` is the *source* server in clone mode, so spreading its
settings copied that server's OAuth client secret, metadata, roots and
behavior flags onto a new entry the user had only given a URL and headers.
Only an edit carries the other fields forward now — it has to, since
`settings` replaces the node wholesale — while add and clone build from the
empty settings shape.

Also drops an unnecessary `as unknown as` in the new useServers test
(`mcpServers` already holds `StoredMCPServer`, which types `headers`), and
removes a `settings: null` instruction the parameter's type does not allow.

Signed-off-by: cliffhall <cliff@futurescale.com>

core/react/useServers.ts

@@ -265,8 +265,8 @@ export function useServers(opts: UseServersOptions): UseServersResult {
       // settings node on disk" — so a config-only save cannot silently wipe
       // persisted headers / metadata / OAuth credentials. A caller editing a
       // field that lives on settings (e.g. ServerConfigModal's custom headers)
-      // passes the full settings object it wants written. To explicitly clear
-      // settings, send `settings: null`.
+      // passes the full settings object it wants written, which replaces the
+      // node wholesale — so it must carry forward the fields it isn't editing.
       const res = await doFetch(
         `${base}/api/servers/${encodeURIComponent(originalId)}`,
         {