fix(mobile): give a direct handshake as long as the desktop allows it (#36864)

The phone gave a direct socket 5 s from open to authenticated, while
the desktop keeps an unauthenticated socket for 10 s. On a slow link
(a handshake is two round trips, so ~2.5 s RTT is enough) the phone
dropped the socket the desktop would have accepted, then redialed into
the same deadline forever: the 'Can't connect' loop a test documented.

Both ends now share PAIRED_E2EE_HANDSHAKE_DEADLINE_MS (10 s). A link
that needs 6 s to authenticate now connects on the first dial; one that
never authenticates still escalates the same way, 5 s later. Relay
dials are unaffected: they keep their own stage budgets and race the
direct dial after 2.5 s.

src/shared/paired-e2ee-handshake-deadline.ts

@@ -0,0 +1,6 @@
+/**
+ * How long a paired socket may take from open to authenticated, on both ends. The desktop closes a
+ * socket that has not authenticated by then, so a phone that gives up sooner drops slow links the
+ * desktop would still have accepted.
+ */
+export const PAIRED_E2EE_HANDSHAKE_DEADLINE_MS = 10_000