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/models/provider.rs

@@ -0,0 +1,74 @@
+use serde::{Deserialize, Serialize};
+
+/// Supported LLM provider backends.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub enum LlmProvider {
+    /// Self-hosted models via Ollama
+    Ollama,
+    /// Hugging Face Inference API
+    HuggingFace,
+    /// OpenAI-compatible endpoints
+    OpenAi,
+    /// Anthropic Claude API
+    Anthropic,
+}
+
+impl LlmProvider {
+    /// Returns the display name for a provider.
+    pub fn label(&self) -> &'static str {
+        match self {
+            Self::Ollama => "Ollama",
+            Self::HuggingFace => "Hugging Face",
+            Self::OpenAi => "OpenAI",
+            Self::Anthropic => "Anthropic",
+        }
+    }
+}
+
+/// A model available from a provider.
+///
+/// # Fields
+///
+/// * `id` - Unique model identifier (e.g. "llama3.1:8b")
+/// * `name` - Human-readable display name
+/// * `provider` - Which provider hosts this model
+/// * `context_window` - Maximum context length in tokens
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct ModelEntry {
+    pub id: String,
+    pub name: String,
+    pub provider: LlmProvider,
+    pub context_window: u32,
+}
+
+/// An embedding model available from a provider.
+///
+/// # Fields
+///
+/// * `id` - Unique embedding model identifier
+/// * `name` - Human-readable display name
+/// * `provider` - Which provider hosts this model
+/// * `dimensions` - Output embedding dimensions
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct EmbeddingEntry {
+    pub id: String,
+    pub name: String,
+    pub provider: LlmProvider,
+    pub dimensions: u32,
+}
+
+/// Active provider configuration state.
+///
+/// # Fields
+///
+/// * `provider` - Currently selected provider
+/// * `selected_model` - ID of the active chat model
+/// * `selected_embedding` - ID of the active embedding model
+/// * `api_key_set` - Whether an API key has been configured
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct ProviderConfig {
+    pub provider: LlmProvider,
+    pub selected_model: String,
+    pub selected_embedding: String,
+    pub api_key_set: bool,
+}