fix(server): unblock FLUX checkout from Electron desktop renderer

The packaged Electron renderer loads from file://, so its Stripe checkout
and portal requests carry no Referer and an opaque/absent Origin.
resolveTrustedRequestOrigin() returned undefined and the routes threw
INVALID_ORIGIN ("Missing trusted request origin"), blocking FLUX purchases
on desktop. Web and mobile were unaffected because they send a trusted web
origin. CORS was not the blocker: file:// requests already reach the API
(balance, providers, characters all work), so only the checkout-specific
redirect-base dependency failed.

Stripe success_url/cancel_url/return_url must be http(s) URLs, so file://
can never be a valid redirect base. Replace the trusted-origin requirement
with resolveCheckoutRedirectBase(), which prefers the request's trusted
origin (web/mobile return to where they started) and falls back to a
configured canonical web app URL (WEB_APP_URL, default https://airi.moeru.ai)
for origin-less clients like the desktop app.

On the client, the desktop app opens checkout in the system browser via
window.open (routed through setWindowOpenHandler -> shell.openExternal)
instead of navigating the file:// window to Stripe, and refreshes the FLUX
balance on window focus since it never receives the success_url redirect.