feat(i18n): add internationalization with DE, FR, ES, PT translations

Add a compile-time i18n system with 270 translation keys across 5 locales
(EN, DE, FR, ES, PT). Translations are embedded via include_str! and parsed
lazily into flat HashMaps with English fallback for missing keys.

- Add src/i18n module with Locale enum, t()/tw() lookup functions, and tests
- Add JSON translation files for all 5 locales under assets/i18n/
- Provide locale Signal via Dioxus context in App, persisted to localStorage
- Replace all hardcoded UI strings across 33 component/page files
- Add compact locale picker (globe icon + ISO alpha-2 code) in sidebar header
- Add click-outside backdrop dismissal for locale dropdown

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

assets/main.css

@@ -70,13 +70,113 @@ h6 {
     top: 0;
 }
 
-/* -- Sidebar Header -- */
+/* -- Sidebar Top Row (header + locale picker) -- */
+.sidebar-top-row {
+    display: flex;
+    align-items: flex-start;
+    justify-content: space-between;
+    padding: 20px 14px 16px 20px;
+    border-bottom: 1px solid var(--border-primary);
+}
+
 .sidebar-header {
     display: flex;
     align-items: center;
     gap: 12px;
-    padding: 24px 20px 20px;
-    border-bottom: 1px solid var(--border-primary);
+    min-width: 0;
+    flex: 1;
+}
+
+/* -- Locale Picker -- */
+.locale-picker {
+    position: relative;
+    flex-shrink: 0;
+    margin-top: 2px;
+}
+
+.locale-picker-btn {
+    display: flex;
+    align-items: center;
+    gap: 4px;
+    padding: 4px 8px;
+    border-radius: 6px;
+    border: 1px solid var(--border-primary);
+    background: transparent;
+    color: var(--text-muted);
+    font-size: 11px;
+    font-weight: 600;
+    font-family: 'Space Grotesk', sans-serif;
+    cursor: pointer;
+    transition: background-color 0.15s ease, color 0.15s ease, border-color 0.15s ease;
+    letter-spacing: 0.5px;
+}
+
+.locale-picker-btn:hover {
+    background-color: var(--bg-surface);
+    color: var(--text-primary);
+    border-color: var(--text-muted);
+}
+
+.locale-picker-code {
+    line-height: 1;
+}
+
+.locale-picker-backdrop {
+    position: fixed;
+    inset: 0;
+    z-index: 49;
+}
+
+.locale-picker-dropdown {
+    position: absolute;
+    top: calc(100% + 4px);
+    right: 0;
+    z-index: 50;
+    min-width: 140px;
+    background-color: var(--bg-sidebar);
+    border: 1px solid var(--border-primary);
+    border-radius: 8px;
+    padding: 4px;
+    box-shadow: 0 8px 24px rgba(0, 0, 0, 0.2);
+}
+
+.locale-picker-item {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    width: 100%;
+    padding: 6px 10px;
+    border: none;
+    border-radius: 6px;
+    background: transparent;
+    color: var(--text-muted);
+    font-size: 13px;
+    cursor: pointer;
+    transition: background-color 0.12s ease, color 0.12s ease;
+    text-align: left;
+}
+
+.locale-picker-item:hover {
+    background-color: var(--bg-surface);
+    color: var(--text-primary);
+}
+
+.locale-picker-item--active {
+    color: var(--accent);
+    background-color: rgba(145, 164, 210, 0.1);
+}
+
+.locale-picker-item-code {
+    font-family: 'Space Grotesk', sans-serif;
+    font-weight: 600;
+    font-size: 11px;
+    letter-spacing: 0.5px;
+    width: 22px;
+    text-align: center;
+}
+
+.locale-picker-item-label {
+    font-weight: 500;
 }
 
 .avatar-circle {