TLS: fingerprint-confirm flow for self-signed gateway certs (issue #14)

docs/09 §9.4 promised a SSH-host-key-style fingerprint confirm on first
pair, but the app built a default OkHttpClient with no certificate
handling — self-signed gateway certs were simply rejected.

Build the flow:
- TlsPinning.kt: PinningTrustManager wraps the platform default trust
  manager; a rejected cert is accepted only when its SHA-256 fingerprint
  matches the user-confirmed pin, anything else fails with
  TlsFingerprintRequired (hostname verification still applies).
- SecureStore.pinnedCertFingerprint (Android EncryptedSharedPreferences +
  desktop settings.json), cleared on forget().
- GatewayClient: pinning socket factory on the shared client (all legs
  inherit it), new terminal State.TlsConfirmRequired, unwrap the nested
  TlsFingerprintRequired in connect loop / watchdog / SSE / poll /
  testHello.
- ConnectScreen: confirm dialog showing the fingerprint ("Confirm &
  pin" re-runs the connect); IrisApp routes TlsConfirmRequired there;
  ChatScreen + desktop tray handle the new state.
- Docs: §9.4 now describes the real flow (incl. SAN requirement), gap
  table item 6 → implemented, setup.md limitation note updated.
- Tests: TlsPinningTest (fingerprint vs openssl, pin accept/reject,
  live pin read, unwrap) + TlsPinningIntegrationTest (real TLS
  handshake: unpinned → confirm data, pinned → 200).

Live E2E verified on the phone: first pair against a self-signed
IRIS_HTTP_CERT gateway shows the dialog, confirm pins, chat works,
auto-reconnect after gateway restart uses the pin.

docs/setup.md

@@ -145,9 +145,12 @@ does nothing) — use ntfy (the default), or add Firebase later.
 - **WSS:** set `IRIS_WS_CERT` / `IRIS_WS_KEY` (paths, in
   `~/.hermes/.env`) and the server serves `wss://` instead of `ws://`.
 
-> **Honest limitation:** the app has **no certificate pinning** yet, so
-> self-signed certs won't work — remote access requires **CA-signed** WSS for
-> now. Plain `ws://` on a trusted LAN (or inside Tailscale) stays the default.
+> **Self-signed certs:** the app has a fingerprint-confirm flow (docs/09
+> §9.4): on first pair it shows the gateway cert's SHA-256 fingerprint; once
+> you confirm it, the cert is pinned in secure storage (like an SSH host
+> key). The cert needs a SAN for the URL host. CA-signed certs work out of
+> the box. Plain `ws://` on a trusted LAN (or inside Tailscale) stays the
+> default.
 
 ## 6. Troubleshooting