feat: add language switching to admin and merchant frontends

- Add cookie to ADMIN resolution chain (cookie → user_pref → "en")
- Add explicit MERCHANT resolution (cookie → user_pref → "fr")
- Add language selector dropdown to admin and merchant headers
- Add languageSelector() function to merchant init-alpine.js
- Add flag-icons CSS and i18n.js setup to merchant base template
- Add compact flag-based language selector to both login pages
- Make lang attribute dynamic on all base and login templates
- Pass current_language to login route template context
- Update architecture doc with ADMIN/MERCHANT resolution priorities

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/architecture/language-i18n.md

@@ -50,7 +50,8 @@ Language resolution varies by frontend type. Each chain is evaluated top-to-bott
 
 | Frontend | Priority (highest → lowest) |
 |----------|----------------------------|
-| **ADMIN** | User `preferred_language` → `"en"` |
+| **ADMIN** | Cookie (`lang`) → User `preferred_language` → `"en"` |
+| **MERCHANT** | Cookie (`lang`) → User `preferred_language` → `"fr"` |
 | **STORE** | Cookie (`lang`) → User `preferred_language` → Store `dashboard_language` → `"fr"` |
 | **STOREFRONT** | Customer `preferred_language` → Cookie (`lang`) → Store `storefront_language` → Browser `Accept-Language` → `"fr"` |
 | **PLATFORM** | Cookie (`lang`) → Browser `Accept-Language` → `"fr"` |
@@ -61,9 +62,10 @@ The **cookie** (`lang`) is set by the language switcher UI via `POST /api/v1/pla
 
 | Context | Language Source | Database Field |
 |---------|-----------------|----------------|
+| Admin Panel | User's `preferred_language` | `users.preferred_language` |
+| Merchant Portal | User's `preferred_language` | `users.preferred_language` |
 | Store Dashboard | Store's `dashboard_language` | `stores.dashboard_language` |
 | Customer Storefront | Store's `storefront_language` | `stores.storefront_language` |
-| Admin Panel | User's `preferred_language` | `users.preferred_language` |
 
 ---
 
@@ -490,12 +492,16 @@ static/locales/
 
 | File | Type | Notes |
 |------|------|-------|
-| `static/shop/js/shop-layout.js` | JS | `languageSelector()` function |
-| `static/store/js/init-alpine.js` | JS | `languageSelector()` function |
+| `static/shop/js/shop-layout.js` | JS | `languageSelector()` for storefront |
+| `app/modules/core/static/store/js/init-alpine.js` | JS | `languageSelector()` for store dashboard |
+| `app/modules/core/static/admin/js/init-alpine.js` | JS | `languageSelector()` for admin |
+| `app/modules/core/static/merchant/js/init-alpine.js` | JS | `languageSelector()` for merchant |
+| `app/templates/store/partials/header.html` | Template | Store dashboard language selector |
+| `app/templates/admin/partials/header.html` | Template | Admin language selector |
+| `app/templates/merchant/partials/header.html` | Template | Merchant language selector |
 | `app/templates/shop/base.html` | Template | Storefront language selector |
-| `app/templates/store/partials/header.html` | Template | Dashboard language selector |
 | `app/modules/core/routes/api/platform.py` | API | Language endpoints (`/api/v1/platform/language/*`) |
-| `middleware/language.py` | Middleware | Language detection |
+| `middleware/language.py` | Middleware | Language detection per frontend type |
 | `static/locales/*.json` | JSON | Translation files |
 
 ---