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/pages/developer/flow.rs

@@ -1,23 +1,26 @@
 use dioxus::prelude::*;
 
+use crate::i18n::{t, Locale};
+
 /// Flow page placeholder for the LangFlow visual workflow builder.
 ///
 /// Shows a "Coming Soon" card with a disabled launch button.
 /// Will eventually integrate with LangFlow for visual flow design.
 #[component]
 pub fn FlowPage() -> Element {
+    let locale = use_context::<Signal<Locale>>();
+    let l = *locale.read();
+
     rsx! {
         section { class: "placeholder-page",
             div { class: "placeholder-card",
                 div { class: "placeholder-icon", "F" }
-                h2 { "Flow Builder" }
+                h2 { "{t(l, \"developer.flow_title\")}" }
                 p { class: "placeholder-desc",
-                    "Design visual AI workflows with LangFlow. \
-                    Drag-and-drop nodes to create data processing pipelines, \
-                    prompt chains, and integration flows."
+                    "{t(l, \"developer.flow_desc\")}"
                 }
-                button { class: "btn-primary", disabled: true, "Launch Flow Builder" }
-                span { class: "placeholder-badge", "Coming Soon" }
+                button { class: "btn-primary", disabled: true, "{t(l, \"developer.launch_flow\")}" }
+                span { class: "placeholder-badge", "{t(l, \"common.coming_soon\")}" }
             }
         }
     }