feat: Phase 4 — off-grid Bitcoin relay, block headers, dead man's switch

- Typed message dispatch in listener (BlockHeader, TxRelay, LightningRelay, Alert, TxConfirmation)
- Base64 encoding for binary payloads over LoRa (fixes NUL byte truncation)
- Compact block header announcements (88 bytes, fits 160-byte LoRa limit)
- Block header announcer: internet nodes auto-announce new blocks to Archy peers
- TX relay: mesh-only nodes can broadcast transactions via internet-connected peers
- Confirmation tracking: relay node monitors 1/3, 2/3, 3/3 confirmations, sends updates back
- Dead man's switch background task with configurable interval and signed alert broadcast
- 6 new RPC endpoints: relay-tx, block-headers, relay-lightning, deadman-status/configure/checkin
- lnd.create-raw-tx: create signed TX without broadcasting (for mesh relay)
- Web5 wallet: offline detection + "Send via mesh?" prompt with auto relay + confirmation polling
- Mesh.vue: Off-Grid Bitcoin tab, Dead Man tab, Send Bitcoin/Lightning buttons
- TX/Lightning relay sends only to Archy peers (not broadcast to all devices)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/mod.rs

@@ -490,6 +490,7 @@ impl RpcHandler {
             "lnd.payinvoice" => self.handle_lnd_payinvoice(params).await,
             "lnd.create-psbt" => self.handle_lnd_create_psbt(params).await,
             "lnd.finalize-psbt" => self.handle_lnd_finalize_psbt(params).await,
+            "lnd.create-raw-tx" => self.handle_lnd_create_raw_tx(params).await,
             "lnd.gettransactions" => self.handle_lnd_gettransactions().await,
             "lnd.connect-info" => self.handle_lnd_connect_info().await,
 
@@ -652,6 +653,13 @@ impl RpcHandler {
             "mesh.outbox" => self.handle_mesh_outbox(params).await,
             "mesh.session-status" => self.handle_mesh_session_status(params).await,
             "mesh.rotate-prekeys" => self.handle_mesh_rotate_prekeys().await,
+            // Phase 4: Off-grid Bitcoin operations
+            "mesh.relay-tx" => self.handle_mesh_relay_tx(params).await,
+            "mesh.block-headers" => self.handle_mesh_block_headers(params).await,
+            "mesh.relay-lightning" => self.handle_mesh_relay_lightning(params).await,
+            "mesh.deadman-status" => self.handle_mesh_deadman_status().await,
+            "mesh.deadman-configure" => self.handle_mesh_deadman_configure(params).await,
+            "mesh.deadman-checkin" => self.handle_mesh_deadman_checkin().await,
 
             // Transport layer (unified routing)
             "transport.status" => self.handle_transport_status().await,