Moves the fee_address/bet_amount_sats config form out of the main test
UI into a dedicated admin.html, served by a GET /admin route
(registered ahead of the StaticFiles mount so it doesn't shadow the
existing GET/PUT /admin/config API). Deliberately not linked from the
test UI in either direction: reachable only by knowing the URL.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
