feat(secrets): company secrets management UI

New /company/settings/secrets page with create, rotate, edit, and delete
flows. Adds a Secrets entry to the company settings sidebar (and tab nav).
Each row shows name, description, version, and time since last rotation;
per-row actions open dialogs for rotate (textarea for new value), edit
(name + description), and delete (confirmation).

Server-side adds secretService.usages() to enumerate agents that reference
a secret via env bindings, and rejects deletion when any usage exists.
The delete dialog reads the blocking usage list from the error body and
renders it inline so the user knows which agents to detach first.

ui/src/api/secrets.ts

@@ -22,4 +22,6 @@ export const secretsApi = {
     data: { name?: string; description?: string | null; externalRef?: string | null },
   ) => api.patch<CompanySecret>(`/secrets/${id}`, data),
   remove: (id: string) => api.delete<{ ok: true }>(`/secrets/${id}`),
+  usages: (id: string) =>
+    api.get<{ agents: { id: string; name: string; envKeys: string[] }[] }>(`/secrets/${id}/usages`),
 };