feat(chat): raise the composer cap to the tier-2 larger-message capacity

The composer byte cap was 236 (one memo). With larger-messages tier 2 a body
can span up to 16 memo outputs in one tx, so kChatBodyMaxBytes rises to 4000
(a touch under the ~4063 protocol max so the composer never produces a body
compose would reject). The compose buffer grows to match, guarded by a
static_assert, and the emoji picker / byte gauge follow the single constant.
sendChatMessage now shows a specific "message too long" toast on a TooLong
compose instead of the generic compose-failed one (English string added; the
8 locales fall back until translated).

Broadcast is unchanged — both chatSendOutputs call sites already iterate the
N outputs. (Lite backend acceptance of >2 duplicate-address 0-value outputs is
the one bit needing a live smoke test, like the other lite broadcast paths.)

Larger-messages tiers 1-2 are now usable end to end on the full node.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/i18n.cpp

@@ -265,6 +265,7 @@ void I18n::loadBuiltinEnglish()
         strings_["chat_toast_lite_busy"] = "A send is already in progress, or no wallet is open.";
         strings_["chat_toast_waiting_reply"] = "Waiting for the contact to reply before you can message them.";
         strings_["chat_toast_compose_failed"] = "Could not compose the message (too long?).";
+        strings_["chat_toast_too_long"] = "Message too long — shorten it and send again.";
         strings_["chat_toast_request_compose_failed"] = "Could not compose the contact request (invalid address / text?).";
         strings_["chat_toast_request_queued"] = "Contact request queued.";
         strings_["chat_toast_need_funds"] = "Need a small shielded balance to send chat (to cover the fee).";