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/tool_card.rs

@@ -1,6 +1,8 @@
-use crate::models::McpTool;
 use dioxus::prelude::*;
 
+use crate::i18n::{t, Locale};
+use crate::models::McpTool;
+
 /// Renders an MCP tool card with name, description, status indicator, and toggle.
 ///
 /// # Arguments
@@ -9,6 +11,9 @@ use dioxus::prelude::*;
 /// * `on_toggle` - Callback fired when the enable/disable toggle is clicked
 #[component]
 pub fn ToolCard(tool: McpTool, on_toggle: EventHandler<String>) -> Element {
+    let locale = use_context::<Signal<Locale>>();
+    let l = *locale.read();
+
     let status_class = format!("tool-status tool-status--{}", tool.status.css_class());
     let toggle_class = if tool.enabled {
         "tool-toggle tool-toggle--on"
@@ -33,9 +38,9 @@ pub fn ToolCard(tool: McpTool, on_toggle: EventHandler<String>) -> Element {
                         move |_| on_toggle.call(id.clone())
                     },
                     if tool.enabled {
-                        "ON"
+                        "{t(l, \"common.on\")}"
                     } else {
-                        "OFF"
+                        "{t(l, \"common.off\")}"
                     }
                 }
             }