Extract optimistic reply shaping into testable module

Move the optimistic reply object construction out of the ReplyThreadForm
React component into a testable service module with unit tests, keeping
the component a thin UI adapter. Behavior is preserved; all unit,
property, and acceptance tests pass.

By refactorer.

test/unit/optimistic-reply.test.js

@@ -0,0 +1,66 @@
+/*
+  Unit tests for the optimistic reply object builder
+  (src/services/optimistic-reply.js).
+
+  The reply form renders an optimistic reply immediately after a successful
+  broadcast, before the thread is refreshed from the network. This module
+  shapes that reply object so the shape is covered by unit tests and the
+  React form stays a thin adapter.
+*/
+
+'use strict'
+
+const test = require('node:test')
+const assert = require('node:assert/strict')
+
+const { buildOptimisticReply } = require('../../src/services/optimistic-reply')
+
+const TXID = 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
+const ADDR = 'bitcoincash:qqlrzp23w08434twmvr4fxw672whkjy0py26r63g3d'
+
+test('builds a reply with a zero reply count and no child replies', () => {
+  const reply = buildOptimisticReply({
+    txid: TXID,
+    addr: ADDR,
+    text: 'hello',
+    seen: 1234,
+    blockHeight: 800000,
+    displayName: null
+  })
+
+  assert.equal(reply.txid, TXID)
+  assert.equal(reply.addr, ADDR)
+  assert.equal(reply.text, 'hello')
+  assert.equal(reply.seen, 1234)
+  assert.equal(reply.blockHeight, 800000)
+  assert.equal(reply.replyCount, 0)
+  assert.deepEqual(reply.replies, [])
+  assert.equal(reply.profile, undefined)
+})
+
+test('attaches a profile when a display name is present', () => {
+  const reply = buildOptimisticReply({
+    txid: TXID,
+    addr: ADDR,
+    text: 'hello',
+    seen: 1234,
+    blockHeight: undefined,
+    displayName: 'Trout'
+  })
+
+  assert.deepEqual(reply.profile, { name: 'Trout' })
+  assert.equal(reply.blockHeight, undefined)
+})
+
+test('preserves an absent block height', () => {
+  const reply = buildOptimisticReply({
+    txid: TXID,
+    addr: ADDR,
+    text: 'hello',
+    seen: 1234,
+    blockHeight: null,
+    displayName: null
+  })
+
+  assert.equal(reply.blockHeight, null)
+})