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>

src/components/chat_input_bar.rs

@@ -1,3 +1,4 @@
+use crate::i18n::{t, Locale};
 use dioxus::prelude::*;
 
 /// Chat input bar with a textarea and send button.
@@ -16,13 +17,16 @@ pub fn ChatInputBar(
     on_send: EventHandler<String>,
     is_streaming: bool,
 ) -> Element {
+    let locale = use_context::<Signal<Locale>>();
+    let l = *locale.read();
+
     let mut input = input_text;
 
     rsx! {
         div { class: "chat-input-bar",
             textarea {
                 class: "chat-input",
-                placeholder: "Type a message...",
+                placeholder: "{t(l, \"chat.type_message\")}",
                 disabled: is_streaming,
                 rows: "1",
                 value: "{input}",