upgrade: adopt V8.21.3 (V8.13→8.21.3: P5 SFTP/S3, P6 multilingual, editor overhaul, enterprise hardening)

Overlay ~150 upstream items onto our V8.12 base, preserving trade-pro
theme, registry tests, and design docs. Re-applied our patch set
(discovery/env_for/blocks.css/type-mapping/theme_color/frontend entries,
test_theme_pro_v3 mods) — all anchors survived upstream evolution.

New platform enhancement (fixes upgrade-introduced regression):
multilingual.site_locale now honors settings_json.locale, plus
PUT /api/i18n/sites/{id}/locale — standalone English sites get
<html lang='en'> under the V8.15 locale-rewrite pipeline.

compose: adopt V8.21.3 (image 8.21.3-dev.1, /api/ready probes, MySQL/S3
env) with project name pinned to siteforge-v8-10 (data volumes).

Tests: 356 passed / 11 failed — all 11 confirmed pre-existing in
pristine V8.21.3 on this host (10 upstream Windows env + our build test
now green after locale fix). Patch registry updated in plan doc.

Co-Authored-By: Claude Code <noreply@anthropic.com>

app/templates/workspace.html

@@ -16,6 +16,7 @@
    <button data-view="design" class="nav-item"><span class="nav-ico">◈</span><span>外观与设计</span></button>
    <button data-view="import" class="nav-item"><span class="nav-ico">⇩</span><span>导入与迁移</span></button>
    <button data-view="build" class="nav-item"><span class="nav-ico">↗</span><span>构建与交付</span></button>
+   <button data-view="languages" class="nav-item"><span class="nav-ico">文</span><span>多语言管理</span></button>
    <button data-view="settings" class="nav-item"><span class="nav-ico">⚙</span><span>网站设置</span></button>
   </div>
   <details class="advanced"><summary><span class="nav-ico">⌘</span>高级工具 <span class="chevron">⌄</span></summary>