fix(register): auto re-register with proxy on every launch

[DocNR]

Summary

Real tester (npub1whale...) hit a silent push-delivery failure today: their signer pubkey had zero APNs tokens registered with the proxy (`[PRIMARY] Event for pubkey 75fbfcd7... — no registered tokens, skipping` repeated 63 times in 4h). Foreground L1 worked because it catches via WebSocket and bypasses APNs entirely; everything else silently failed. Their workaround was "toggle Clave to foreground every time" — fine as a stopgap, broken as a product.

Root cause: registration with the proxy only happens in `AppState.importKey()` / `generateKey()` and the manual Settings → Register button. Three real failure modes leave the proxy with a stale or absent token:

1. iOS rotates the device token (Apple does this periodically, especially after iOS upgrades). New token from `didRegisterForRemoteNotificationsWithDeviceToken` was being saved to UserDefaults but never pushed to the proxy.
2. User reinstalls Clave from TestFlight — fresh APNs token, but the existing nsec in Keychain means we never re-hit the importKey/generateKey path.
3. Proxy-side token loss (the 2026-04-20 tokens.json migration wiped legacy entries; future bugs may do similar) — no recovery without manual Settings tap.

The previous design comment in `ClaveApp.swift` claimed AppDelegate "doesn't have access to the signer nsec without duplicating LightEvent.signNip98 logic" — true, but solvable by signaling AppState via NotificationCenter and letting it do the actual NIP-98-signed POST.

Changes

• `Clave/ClaveApp.swift` `didRegisterForRemoteNotificationsWithDeviceToken` now posts `.apnsDeviceTokenAvailable` every time iOS hands us a token. New Notification.Name added to the existing extension.
• `Clave/AppState.swift`:
  • `init` adds an observer for `.apnsDeviceTokenAvailable` that updates `deviceToken` and calls `registerWithProxy()` if `isKeyImported`.
  • `loadState()` calls `registerWithProxy()` if both key and stored token are present. Belt-and-suspenders for the cold-launch ordering case where iOS handed us the token via the AppDelegate path before `loadState()` ran (so the observer's `isKeyImported` check failed at that moment because the key hadn't loaded from Keychain yet).

Both paths are idempotent on the proxy side (the /register endpoint upserts), so the common case of "token+pubkey unchanged" is harmless.

Test plan

• `xcodebuild -scheme Clave -destination 'generic/platform=iOS' build` → BUILD SUCCEEDED
• `xcodebuild test` on iPhone 17 Pro Max sim (iOS 26.4) → TEST SUCCEEDED
• Device test (build 26 TestFlight) — reproduce the tester's symptom + verify recovery:
  • On a phone with no current token entry on the proxy (force-removable via `sg clave-proxy -c 'cat /opt/clave-proxy/tokens.json' | jq` to confirm), launch build 26.
  • Within ~1s of launch, check `curl https://proxy.clave.casa/health | jq .total_tokens` — should increment by 1.
  • Send a sign request from a paired client with Clave force-closed → NSE wakes → request signs.
• Real tester confirmation: ship build 26 → ask npub1whale... to install → check whether their 75fbfcd7... pubkey appears in tokens.json without them tapping Settings → Register.

Closes

BACKLOG: "Auto-register on launch (low priority): loadState() doesn't re-register with the proxy — only importKey()/generateKey() do." Promoted to High after this incident.

Why this didn't make build 25

Build 25 just shipped (PR #13 merged + archived). This is a follow-up for build 26. Independent enough to merge separately; no conflicts with PR #14 (proxy resilience, server-side).

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