fix(secrets): drop fork's CompanySecrets page and dedupe sidebar/route

The merge of upstream's #5429 (provider vaults + AWS Secrets Manager) added a
new 2,155-line Secrets.tsx page covering everything the fork's 528-line
CompanySecrets page did, plus vault management and AWS import. Auto-merge took
both sides' sidebar entry and route registration, so React Router picked the
fork's older page first and upstream's AWS UI never rendered.

- ui/src/App.tsx: drop CompanySecrets import + duplicate route at /company/settings/secrets
- ui/src/components/CompanySettingsSidebar.tsx: drop duplicate Secrets nav item
- ui/src/pages/CompanySecrets.tsx: delete (superseded)

Server-side delete guard (services/secrets.ts: refuse delete when secret is
referenced by agents or skills) is preserved and still enforced; upstream's
page surfaces the API error message in place of the fork's inline reference list.

ui/src/components/CompanySettingsSidebar.tsx

@@ -69,7 +69,6 @@ export function CompanySettingsSidebar() {
             end
           />
           <SidebarNavItem to="/company/settings/invites" label="Invites" icon={MailPlus} end />
-          <SidebarNavItem to="/company/settings/secrets" label="Secrets" icon={KeyRound} end />
         </div>
       </nav>
     </aside>