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/static/workspace.css

@@ -2,4 +2,5 @@
 
 /* V8.12: task progress and dangerous actions, consistent focus and mobile sizing */
 .btn-danger{border:1px solid #f1c3c7;color:#a92840;background:#fff5f6}.btn-danger:hover{background:#feecee}.btn:disabled{opacity:.55;cursor:not-allowed}.build-row{min-width:0}.build-meta{overflow-wrap:anywhere}.site-card:focus-visible,.quick-card:focus-visible{outline:3px solid #80aaf6;outline-offset:2px}@media(max-width:570px){.build-row{align-items:flex-start;flex-wrap:wrap}.build-row>.inline-actions{width:100%;padding-left:0}.site-picker-wrap select{max-width:130px}}
+
 .theme-preview.trade-pro{background:linear-gradient(125deg,#0b3d91,#3d6dc1);color:#e8f1fb}.site-icon.trade-pro{background:#e8f1fb;color:#0b3d91}