docs: 交付文档、维护指引与端到端回归

补齐 README、开发计划、维护交接与 E2E 用例，执行最终验收。

frontend/src/__tests__/utils/whatsappLanguages.spec.ts

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+/*
+Copyright 2024-2026 ChatterMate
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { describe, it, expect } from 'vitest'
+import {
+  WHATSAPP_LANGUAGES,
+  DEFAULT_LANGUAGE,
+  languageLabel,
+} from '../../utils/whatsappLanguages'
+
+describe('WHATSAPP_LANGUAGES', () => {
+  it('carries the full list Meta publishes', () => {
+    // Meta listed 111 at transcription time. A merge that silently drops half
+    // the list would otherwise pass every other assertion here.
+    expect(WHATSAPP_LANGUAGES.length).toBe(111)
+  })
+
+  it('has no duplicate codes', () => {
+    const codes = WHATSAPP_LANGUAGES.map((l) => l.code)
+    expect(new Set(codes).size).toBe(codes.length)
+  })
+
+  it('has no duplicate labels', () => {
+    const labels = WHATSAPP_LANGUAGES.map((l) => l.label)
+    expect(new Set(labels).size).toBe(labels.length)
+  })
+
+  it('uses codes in the shapes Meta actually publishes', () => {
+    // Deliberately loose on length: `fil`/`prs_AF` are 3-letter languages and
+    // `es_CRI` a 3-letter region, so a strict xx / xx_YY rule rejects valid codes.
+    for (const { code } of WHATSAPP_LANGUAGES) {
+      expect(code, code).toMatch(/^[a-z]{2,3}(_[A-Z]{2,3})?$/)
+    }
+  })
+
+  it('keeps the irregular codes Meta actually uses', () => {
+    const codes = WHATSAPP_LANGUAGES.map((l) => l.code)
+    // Each of these breaks a naive assumption about the code format.
+    expect(codes).toEqual(
+      expect.arrayContaining(['fil', 'nb', 'prs_AF', 'es_CRI', 'zh_HK', 'si_LK', 'ky_KG', 'rw_RW']),
+    )
+  })
+
+  it('is sorted by label so the picker reads alphabetically', () => {
+    const labels = WHATSAPP_LANGUAGES.map((l) => l.label)
+    expect(labels).toEqual([...labels].sort((a, b) => a.localeCompare(b)))
+  })
+
+  it('has a non-empty label for every entry', () => {
+    for (const { code, label } of WHATSAPP_LANGUAGES) {
+      expect(label.trim(), code).not.toBe('')
+    }
+  })
+})
+
+describe('DEFAULT_LANGUAGE', () => {
+  it('is selectable in the picker', () => {
+    expect(WHATSAPP_LANGUAGES.some((l) => l.code === DEFAULT_LANGUAGE)).toBe(true)
+  })
+})
+
+describe('languageLabel', () => {
+  it('names a known code', () => {
+    expect(languageLabel('en_US')).toBe('English (US)')
+    expect(languageLabel('pt_BR')).toBe('Portuguese (BR)')
+  })
+
+  it('falls back to the raw code', () => {
+    // A template created elsewhere may use a language Meta added after this
+    // list was written — showing the code beats showing nothing.
+    expect(languageLabel('xx_YY')).toBe('xx_YY')
+  })
+})