admin: read admin_path_prefix from /auth/me + build admin URLs from it

[mastermanas805]

Summary

Pairs with api PR #50. The admin customer-management endpoints now register on the agent API under /api/v1/<prefix>/customers/... where <prefix> is a 32+ char alphanumeric secret with the same blast radius as a session token. This PR teaches the dashboard to read the prefix from /auth/me and build the URLs from it client-side.

Changes:

• AuthMeResponse extended with admin_path_prefix?: string.
• fetchMe() stashes the value in a module-local var; logout() clears it.
• getAdminPathPrefix() + setAdminPathPrefix() are the only public read/write seams; buildAdminURL() is the single point that turns the stash into a request path.
• The 4 admin API functions (listAdminCustomers, getAdminCustomer, setAdminCustomerTier, issueAdminCustomerPromo) call buildAdminURL() instead of hardcoding /api/v1/admin/. With no prefix loaded, every builder throws APIError{status:403, code:"admin_endpoints_unavailable"}.
• AdminCustomersPage's route gate is now is_platform_admin && admin_path_prefix (belt-and-braces — the flag without a prefix would render a broken page).
• AppShell hides the sidebar link under the same compound condition.
• The prefix is treated as a credential: never logged, never echoed into rendered UI text.

Test plan

• npm test — 382/382 unit tests pass (17 test files); 6 new tests in src/api/index.test.ts:
  • fetchMe stashes admin_path_prefix when the API serves it
  • fetchMe leaves the prefix empty when the API omits the field
  • logout clears the stashed admin prefix
  • admin builders mint /api/v1/<prefix>/customers when the prefix is set
  • admin builders throw admin_endpoints_unavailable when the prefix is empty
  • admin builders never include /admin/ in the request URL (covers the "prefix contains 'admin' substring" edge case)
• npx tsc --noEmit — clean
• AdminCustomersPage.test.tsx mock updated to surface admin_path_prefix so the page gate sees both signals
• Manual: deploy + admin user verifies the page loads with a fresh /auth/me; non-admin user verifies the sidebar link is hidden and direct navigation to /app/admin/customers redirects to /

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