fix(billing): billing UX hardening — paused/unpaid status, Stripe host whitelist, useMeter errors, poll abort flag, FR diacritics #4078

Problem

Audit of the billing module surfaced 5 high/medium UX reliability issues:

1. paused and unpaid subscription statuses fall through to default grey chip with no action button — user silently stuck
2. Stripe redirect URLs validated only for HTTPS, no host whitelist — malicious/misconfigured API response could redirect off-Stripe
3. useMeter silent .catch(() => {}) on refresh and initial fetch — errors swallowed, no visibility
4. Checkout poll (pollSubscription) has no abort flag — recursive setTimeout chain continues for one tick after beforeUnmount
5. All French diacritics stripped from billing lang/fr.js — rendering broken for FR locale users

Expected behaviour

• paused → warning chip + Reactivate CTA; unpaid → error chip + Update payment method CTA
• All Stripe redirect URLs validated against hostname whitelist (checkout.stripe.com, billing.stripe.com)
• useMeter errors logged and exposed via reactive meterError ref
• Poll aborted cleanly when component unmounts
• All French strings render correctly with proper accents