feat(i18n): add internationalization with DE, FR, ES, PT translations (#12)

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>

Co-authored-by: Sharang Parnerkar <parnerkarsharang@gmail.com>
Reviewed-on: #12

src/components/chat_model_selector.rs

@@ -1,3 +1,4 @@
+use crate::i18n::{t, Locale};
 use dioxus::prelude::*;
 
 /// Dropdown bar for selecting the LLM model for the current chat session.
@@ -17,9 +18,14 @@ pub fn ChatModelSelector(
     available_models: Vec<String>,
     on_change: EventHandler<String>,
 ) -> Element {
+    let locale = use_context::<Signal<Locale>>();
+    let l = *locale.read();
+
     rsx! {
         div { class: "chat-model-bar",
-            label { class: "chat-model-label", "Model:" }
+            label { class: "chat-model-label",
+                "{t(l, \"chat.model_label\")}"
+            }
             select {
                 class: "chat-model-select",
                 value: "{selected_model}",
@@ -34,7 +40,9 @@ pub fn ChatModelSelector(
                     }
                 }
                 if available_models.is_empty() {
-                    option { disabled: true, "No models available" }
+                    option { disabled: true,
+                        "{t(l, \"chat.no_models\")}"
+                    }
                 }
             }
         }