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/theme_store.py

@@ -8,10 +8,10 @@ from __future__ import annotations
 import io, json, re, zipfile, shutil
 from pathlib import Path
 from . import db
-from .core import THEMES as BUILTIN
 
 BASE=Path(__file__).resolve().parent.parent/'themes'
 THEME_ROOT=db.DATA/'theme_packages'
+from .core import THEMES as BUILTIN
 THEME_ID=re.compile(r'^[a-z][a-z0-9-]{2,39}$')
 VERSION=re.compile(r'^\d+\.\d+\.\d+$')