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>

tests/test_theme_registry.py

@@ -105,6 +105,9 @@ def test_trade_pro_builds_real_site(factory):
     assert factory.post(f'/api/factory/sites/{sid}/bootstrap').status_code == 200
     selected = factory.put(f'/api/factory/sites/{sid}/theme', json={'theme': 'trade-pro', 'version': '1.0.0'})
     assert selected.status_code == 200, selected.text
+    # V8.15+ multilingual rewrites <html lang> per site locale — declare English
+    loc = factory.put(f'/api/i18n/sites/{sid}/locale', json={'locale': 'en'})
+    assert loc.status_code == 200, loc.text
     result = factory.post(f'/api/sites/{sid}/builds', json={})
     assert result.status_code == 200, result.text
     bid = result.json()['version']