feat(search-ui): add i18n locales field to SearchUiIndexConfig (plan §13.21)

- Add `locales` field to SearchUiIndexConfig (HashMap<lang, translations>)
- Enable operators to configure custom translations via config endpoint
- JavaScript already has i18n support (lang query param, fallback to en)
- Add documentation for operators on how to configure locales

Acceptance: GET /ui/search/{index}?lang=fr returns French UI strings when
fr locale configured; falls back to en.

crates/miroir-proxy/src/routes/search_ui.rs

@@ -89,6 +89,9 @@ pub struct SearchUiIndexConfig {
     /// Thumbnail field for images
     #[serde(skip_serializing_if = "Option::is_none")]
     pub thumbnail_field: Option<String>,
+    /// i18n locales (lang code -> translation object) for operator-supplied translations (plan §13.21)
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub locales: Option<std::collections::HashMap<String, serde_json::Value>>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -458,6 +461,7 @@ pub async fn get_config(
         primary_key_field: None,
         hit_url_template: None,
         thumbnail_field: None,
+        locales: None,
     }))
 }
 

docs/search_ui_i18n.md

@@ -0,0 +1,93 @@
+# Search UI i18n (Internationalization)
+
+## Overview
+
+The Search UI supports internationalization (i18n) for operator-supplied translations. This is part of plan §13.21.
+
+## How It Works
+
+1. **Query Parameter**: Users can specify a language via the `lang` query parameter:
+   - Example: `GET /ui/search/myindex?lang=fr`
+
+2. **Browser Fallback**: If no `lang` parameter is provided, the UI uses `navigator.language`
+
+3. **Fallback to English**: If the requested language is not configured, the UI falls back to English (the default built-in locale)
+
+## Configuring Custom Locales
+
+Operators can add custom translations via the `POST /_miroir/ui/search/{index}/config` endpoint:
+
+```bash
+curl -X POST "http://miroir:8080/_miroir/ui/search/myindex/config" \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer <admin_key>" \
+  -d '{
+    "locales": {
+      "fr": {
+        "ui.title": "Recherche",
+        "ui.logo": "Recherche",
+        "ui.search.placeholder": "Rechercher...",
+        "ui.search.label": "Recherche",
+        "ui.search.ariaLabel": "Requête de recherche",
+        "ui.search.hint": "Tapez pour rechercher. Appuyez sur / pour focus. Utilisez les flèches pour naviguer.",
+        "ui.search.button": "Rechercher",
+        "ui.filters": "Filtres",
+        "ui.sort.label": "Trier:",
+        "ui.sort.relevance": "Pertinence",
+        "ui.perPage.label": "Par page:",
+        "ui.results.empty": "Aucun résultat trouvé",
+        "ui.results.emptyHint": "Essayez d'ajuster votre recherche ou vos filtres",
+        "ui.results.count": "{count} résultats ({time}ms)"
+      }
+    }
+  }'
+```
+
+## Built-in Translations
+
+The Search UI includes English translations by default in the bundled JavaScript. The `t()` function handles variable interpolation:
+
+```javascript
+// With interpolation
+t('ui.results.count', { count: 42, time: 15 })
+// Returns: "42 results (15ms)"
+
+// With French locale
+// Returns: "42 résultats (15ms)"
+```
+
+## Translation Keys
+
+The following translation keys are supported (with default English values):
+
+### UI Labels
+- `ui.title` - Page title
+- `ui.logo` - Logo text
+- `ui.search.placeholder` - Search input placeholder
+- `ui.search.label` - Search label
+- `ui.search.ariaLabel` - Search ARIA label
+- `ui.search.hint` - Search keyboard hint
+- `ui.search.button` - Search button ARIA label
+- `ui.darkMode.ariaLabel` - Dark mode toggle ARIA label
+- `ui.filters` - Filters button text
+- `ui.sort.label` - Sort selector label
+- `ui.sort.relevance` - Default sort option text
+- `ui.perPage.label` - Per-page selector label
+
+### Results
+- `ui.results.empty` - Empty state title
+- `ui.results.emptyHint` - Empty state hint
+- `ui.results.didYouMean` - "Did you mean" suggestion (use `{query}` variable)
+- `ui.results.count` - Result count (use `{count}` and `{time}` variables)
+
+### Errors
+- `ui.error.noIndex` - No index specified error
+- `ui.error.searchFailed` - Search failed error (use `{status}` variable)
+- `ui.error.initFailed` - Init failed error (use `{message}` variable)
+- `ui.error.sessionFailed` - Session fetch failed
+
+### Accessibility
+- `ui.filters.ariaLabel` - Filters ARIA label
+- `ui.pagination.ariaLabel` - Pagination ARIA label
+- `ui.keyboardShortcuts.ariaLabel` - Keyboard shortcuts ARIA label
+- `ui.keyboardShortcuts.text` - Keyboard shortcuts help text