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_action_bar.rs

@@ -1,3 +1,4 @@
+use crate::i18n::{t, Locale};
 use dioxus::prelude::*;
 use dioxus_free_icons::icons::fa_solid_icons::{FaCopy, FaPenToSquare, FaShareNodes};
 
@@ -22,6 +23,9 @@ pub fn ChatActionBar(
     has_assistant_message: bool,
     has_user_message: bool,
 ) -> Element {
+    let locale = use_context::<Signal<Locale>>();
+    let l = *locale.read();
+
     if !has_messages {
         return rsx! {};
     }
@@ -31,34 +35,34 @@ pub fn ChatActionBar(
             button {
                 class: "chat-action-btn",
                 disabled: !has_assistant_message,
-                title: "Copy last response",
+                title: "{t(l, \"chat.copy_response\")}",
                 onclick: move |_| on_copy.call(()),
                 dioxus_free_icons::Icon {
                     icon: FaCopy,
                     width: 14, height: 14,
                 }
-                span { class: "chat-action-label", "Copy" }
+                span { class: "chat-action-label", "{t(l, \"common.copy\")}" }
             }
             button {
                 class: "chat-action-btn",
-                title: "Copy conversation",
+                title: "{t(l, \"chat.copy_conversation\")}",
                 onclick: move |_| on_share.call(()),
                 dioxus_free_icons::Icon {
                     icon: FaShareNodes,
                     width: 14, height: 14,
                 }
-                span { class: "chat-action-label", "Share" }
+                span { class: "chat-action-label", "{t(l, \"common.share\")}" }
             }
             button {
                 class: "chat-action-btn",
                 disabled: !has_user_message,
-                title: "Edit last message",
+                title: "{t(l, \"chat.edit_last\")}",
                 onclick: move |_| on_edit.call(()),
                 dioxus_free_icons::Icon {
                     icon: FaPenToSquare,
                     width: 14, height: 14,
                 }
-                span { class: "chat-action-label", "Edit" }
+                span { class: "chat-action-label", "{t(l, \"common.edit\")}" }
             }
         }
     }