feat(ui): add dashboard sections with sidebar navigation and mock views

Add seven sidebar sections (Dashboard, Providers, Chat, Tools,
Knowledge Base, Developer, Organization) with fully rendered mock views,
nested sub-shells for Developer and Organization, and SearXNG container
for future news feed integration. Replaces the previous OverviewPage
with a news feed dashboard.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/sub_nav.rs

@@ -0,0 +1,44 @@
+use crate::app::Route;
+use dioxus::prelude::*;
+
+/// A single tab item for the sub-navigation bar.
+///
+/// # Fields
+///
+/// * `label` - Display text for the tab
+/// * `route` - Route to navigate to when clicked
+#[derive(Clone, PartialEq)]
+pub struct SubNavItem {
+    pub label: &'static str,
+    pub route: Route,
+}
+
+/// Horizontal tab navigation bar used inside nested shell layouts.
+///
+/// Highlights the active tab based on the current route.
+///
+/// # Arguments
+///
+/// * `items` - List of tab items to render
+#[component]
+pub fn SubNav(items: Vec<SubNavItem>) -> Element {
+    let current_route = use_route::<Route>();
+
+    rsx! {
+        nav { class: "sub-nav",
+            for item in &items {
+                {
+                    let is_active = item.route == current_route;
+                    let class = if is_active { "sub-nav-item sub-nav-item--active" } else { "sub-nav-item" };
+                    rsx! {
+                        Link {
+                            class: "{class}",
+                            to: item.route.clone(),
+                            "{item.label}"
+                        }
+                    }
+                }
+            }
+        }
+    }
+}