feat(frontend): implement GetMempoolStream for 0-conf lightwallet txs

The CompactTxStreamer service did not implement any mempool RPC, so
lightwallets (SilentDragonXLite / ObsidianDragonLite) received UNIMPLEMENTED
from get_mempool_stream and permanently stopped their mempool monitor. As a
result the lite wallet could not see 0-confirmation transactions and only
surfaced incoming shielded chat messages after they were mined (~1 block).

Add GetMempoolStream(Empty) returns (stream RawTransaction). A single
process-wide monitor (frontend/mempool.go) polls the node's getrawmempool and
fetches each new tx via getrawtransaction (full serialized bytes, so shielded
memos are preserved -- CompactTx's 52-byte prefix would not carry a memo), then
fans the tx out to every subscribed stream. This avoids polling the node once
per connected wallet: N clients share one poll loop. The monitor starts lazily
on the first subscription, idles without touching the node when no clients are
connected, and closes all subscriber streams when a new block is mined (the
semantics the client's monitor loop expects; the client re-syncs and reconnects).

Purely additive and wire-compatible: RawTransaction field numbers and the
cash.z.wallet.sdk.rpc package match the deployed client; existing clients that
never call GetMempoolStream are unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

walletrpc/service.proto

@@ -87,6 +87,13 @@ service CompactTxStreamer {
     // t-Address support
     rpc GetAddressTxids(TransparentAddressBlockFilter) returns (stream RawTransaction) {}
 
+    // Mempool
+    // Return a stream of current mempool transactions as full RawTransactions. The stream stays
+    // open while there are mempool transactions and is closed when a new block is mined, at which
+    // point the client re-syncs the block and reconnects. Full RawTransactions (not CompactTx) are
+    // required so wallets can read shielded memos of 0-confirmation transactions (e.g. chat).
+    rpc GetMempoolStream(Empty) returns (stream RawTransaction) {}
+
     // Misc
     rpc GetLightdInfo(Empty) returns (LightdInfo) {}
     rpc GetCoinsupply(Empty) returns (Coinsupply) {}