Switch Nym WS transport to nym-client's binary protocol, drop second base64 layer

Client.Send now speaks nym-client's native binary websocket protocol
(tag || recipient || conn_id || data_len || data) instead of wrapping
the ciphertext in a JSON text message, which required base64-encoding
it a second time on top of the browser's own base64 layer.

The reader's self-address query also switched to the binary protocol:
nym-client picks text-vs-binary for every later "received" push based
on the format of the last request seen on a connection, so leaving the
reader in JSON/text mode would have made nym-client run a lossy UTF-8
conversion over the now-unencoded binary payload, corrupting it. Fixed
the binary Received-frame parsing along the way (previous code assumed
a fixed 16-byte tag with no length prefix, which never matched the real
protocol and was never exercised while the connection stayed text-mode).

Verified end-to-end against the real embedded nym-client 1.1.76 binary,
with the exact wire format cross-checked against upstream nym source at
the pinned build commit.

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