perf: fast relay connect on mobile and desktop + connect splash + edit-safe instances

Relay connect used to serialize a dead LAN probe (up to 8s per stale address
on mobile, 2-4s on desktop) in front of the relay attempt, then paid a second
WebSocket connect + E2EE handshake because the probe tunnel was thrown away.

- mobile probeConnectionCandidates: race the relay probe against the direct
  chain with a 1.5s direct headstart; a live LAN still wins, a dead one no
  longer delays startup
- relay probes adopt their tunnel as the runtime tunnel (adoptRelayTunnel)
  instead of dialing a fresh one — applies to auto-connect, pairing redeem,
  password login, and the desktop host switcher's relay fallback
- relay probe drops the /health round-trip: the E2EE handshake already proves
  the server identity, /auth/session alone proves liveness and auth
- desktop restoreDesktopRelayRuntime: same headstart race; a late direct
  success hot-switches back (stable runtimeKey); startup probe now passes
  expectedServerId so a re-leased LAN address never sees the token
- launch splash shows 'Connecting to device: <label>' with animated dots
  under the (still centered) logo, translated in all locales
- editing a saved instance no longer rebuilds it from the URL field alone:
  the id is passed through, relay/https candidates are preserved, and a
  token-key change migrates the Keychain token instead of orphaning it

packages/ui/src/lib/i18n/messages/fr.ts

@@ -2564,6 +2564,7 @@ export const dict = {
   'mobile.connect.saved.empty': 'Aucune connexion enregistrée pour le moment.',
   'mobile.connect.relay.badge': 'via OpenChamber Relay',
   'mobile.connect.error.urlRequired': 'Saisissez une URL de serveur.',
+  'mobile.connect.splash.connectingTo': 'Connexion à l’appareil :',
   'mobile.connect.error.invalidUrl': 'Cette URL de serveur n\'est pas valide.',
   'mobile.connect.error.unreachable': 'Impossible de joindre ce serveur OpenChamber.',
   'mobile.connect.error.authRequired': 'Ce serveur nécessite un mot de passe ou un jeton client.',