feat(i18n): add browser-language localization with Jinja2 _() and locale middleware

Add i18n support to the blog web UI with 4 languages (en/ru/fr/de),
80 translation keys, automatic Accept-Language detection, persistent
locale cookie, and a language switcher dropdown in the header.

- Infrastructure: TranslationService, translation dicts, convenience _()
- Presentation: locale middleware, /web/lang/{locale} switcher route
- Templates: all 9 templates use {{ _(key, current_locale) }}
- Tests: 26 tests across TranslationService, locale detection helpers
- Docs: TEST_MODEL.md and FEATURE_INFRASTRUCTURE.md updated with TC-UNIT-811-821

app/presentation/templates/pages/profile.html

@@ -4,7 +4,7 @@
 
 {% block content %}
 <div class="page-header" data-testid="page-header-profile">
-    <h1 class="page-title" data-testid="page-title-profile">User Profile</h1>
+    <h1 class="page-title" data-testid="page-title-profile">{{ _('profile.title', current_locale) }}</h1>
 </div>
 
 <div class="card" data-testid="profile-card">
@@ -25,18 +25,18 @@
 
         <div class="profile-details" data-testid="profile-details">
             <div class="profile-field" data-testid="profile-field-email">
-                <span class="profile-label" data-testid="profile-label-email">Email:</span>
-                <span class="profile-value" data-testid="profile-value-email">{{ user.email or 'Not provided' }}</span>
+                <span class="profile-label" data-testid="profile-label-email">{{ _('profile.email', current_locale) }}</span>
+                <span class="profile-value" data-testid="profile-value-email">{{ user.email or _('profile.not_provided', current_locale) }}</span>
             </div>
 
             <div class="profile-field" data-testid="profile-field-userid">
-                <span class="profile-label" data-testid="profile-label-userid">User ID:</span>
+                <span class="profile-label" data-testid="profile-label-userid">{{ _('profile.user_id', current_locale) }}</span>
                 <span class="profile-value" data-testid="profile-value-userid">{{ user.user_id }}</span>
             </div>
 
             {% if user.first_name or user.last_name %}
             <div class="profile-field" data-testid="profile-field-name">
-                <span class="profile-label" data-testid="profile-label-name">Name:</span>
+                <span class="profile-label" data-testid="profile-label-name">{{ _('profile.name', current_locale) }}</span>
                 <span class="profile-value" data-testid="profile-value-name">
                     {{ user.first_name or '' }} {{ user.last_name or '' }}
                 </span>
@@ -51,7 +51,7 @@
                 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
                     <path d="M10 12L6 8L10 4" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                 </svg>
-                Back to Home
+                {{ _('profile.back_home', current_locale) }}
             </a>
 
             {% if can_create %}
@@ -59,7 +59,7 @@
                 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" style="margin-right: 0.5rem;">
                     <path d="M8 2v12M2 8h12" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                 </svg>
-                New Post
+                {{ _('profile.new_post', current_locale) }}
             </a>
             {% endif %}
         </div>