feat: Phase 3 Week 4 — mesh RPC endpoints for typed messages + session management

Backend (6 new RPC endpoints):
- mesh.send-invoice: create Lightning invoice, send bolt11 to mesh peer
- mesh.send-coordinate: send GPS coordinates (integer microdegrees)
- mesh.send-alert: send signed emergency alert (with optional GPS)
- mesh.outbox: list pending store-and-forward messages
- mesh.session-status: get Double Ratchet session info per peer
- mesh.rotate-prekeys: force X3DH prekey rotation

Mock backend: matching dev mode responses for all 6 new endpoints

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

neode-ui/mock-backend.js

@@ -1570,6 +1570,95 @@ app.post('/rpc/v1', (req, res) => {
         return res.json({ result: { configured: true } })
       }
 
+      case 'mesh.send-invoice': {
+        console.log(`[Mesh] Send invoice: ${params?.amount_sats} sats to contact ${params?.contact_id}`)
+        return res.json({
+          result: {
+            sent: true,
+            message_id: Math.floor(Math.random() * 10000) + 200,
+            amount_sats: params?.amount_sats,
+            bolt11: `lnbc${params?.amount_sats}n1pjmesh...`,
+          },
+        })
+      }
+
+      case 'mesh.send-coordinate': {
+        console.log(`[Mesh] Send coordinate: ${params?.lat}, ${params?.lng} to contact ${params?.contact_id}`)
+        return res.json({
+          result: {
+            sent: true,
+            message_id: Math.floor(Math.random() * 10000) + 300,
+            lat: Math.round((params?.lat || 0) * 1000000),
+            lng: Math.round((params?.lng || 0) * 1000000),
+          },
+        })
+      }
+
+      case 'mesh.send-alert': {
+        console.log(`[Mesh] Send alert: ${params?.alert_type} — ${params?.message}`)
+        return res.json({
+          result: {
+            sent: true,
+            alert_type: params?.alert_type || 'status',
+            signed: true,
+          },
+        })
+      }
+
+      case 'mesh.outbox': {
+        return res.json({
+          result: {
+            messages: [
+              {
+                id: 1,
+                dest_did: 'did:key:z6MkrHKPxJP6tvCvXMaJKZd3rRA2Y44tyftVhR8FDCMKGFjb',
+                from_did: 'did:key:z6MkSelf',
+                created_at: new Date(Date.now() - 1800000).toISOString(),
+                ttl_secs: 86400,
+                retry_count: 3,
+                relay_hops: 0,
+                expired: false,
+              },
+              {
+                id: 2,
+                dest_did: 'did:key:z6MknGc3ocHs3zdPiJbnaaqDi58NGb4pk1Sp7NQD5EjEREWh',
+                from_did: 'did:key:z6MkSelf',
+                created_at: new Date(Date.now() - 7200000).toISOString(),
+                ttl_secs: 86400,
+                retry_count: 8,
+                relay_hops: 1,
+                expired: false,
+              },
+            ],
+            count: 2,
+          },
+        })
+      }
+
+      case 'mesh.session-status': {
+        const hasSess = (params?.contact_id === 1 || params?.contact_id === 4)
+        return res.json({
+          result: {
+            has_session: hasSess,
+            forward_secrecy: hasSess,
+            message_count: hasSess ? 23 : 0,
+            ratchet_generation: hasSess ? 7 : 0,
+            peer_did: hasSess ? 'did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2ReMkBe4bR6XBIDNq9' : null,
+          },
+        })
+      }
+
+      case 'mesh.rotate-prekeys': {
+        console.log('[Mesh] Rotating prekeys...')
+        return res.json({
+          result: {
+            rotated: true,
+            signed_prekey_id: Math.floor(Math.random() * 1000000),
+            one_time_prekeys: 10,
+          },
+        })
+      }
+
       // =====================================================================
       // Transport Layer (unified routing: mesh > lan > tor)
       // =====================================================================