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/organization.rs

@@ -0,0 +1,84 @@
+use serde::{Deserialize, Serialize};
+
+/// Role assigned to an organization member.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub enum MemberRole {
+    /// Full administrative access
+    Admin,
+    /// Standard user access
+    Member,
+    /// Read-only access
+    Viewer,
+}
+
+impl MemberRole {
+    /// Returns the display label for a member role.
+    pub fn label(&self) -> &'static str {
+        match self {
+            Self::Admin => "Admin",
+            Self::Member => "Member",
+            Self::Viewer => "Viewer",
+        }
+    }
+
+    /// Returns all available roles for populating dropdowns.
+    pub fn all() -> &'static [Self] {
+        &[Self::Admin, Self::Member, Self::Viewer]
+    }
+}
+
+/// A member of the organization.
+///
+/// # Fields
+///
+/// * `id` - Unique member identifier
+/// * `name` - Display name
+/// * `email` - Email address
+/// * `role` - Assigned role within the organization
+/// * `joined_at` - ISO 8601 join date
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct OrgMember {
+    pub id: String,
+    pub name: String,
+    pub email: String,
+    pub role: MemberRole,
+    pub joined_at: String,
+}
+
+/// A pricing plan tier.
+///
+/// # Fields
+///
+/// * `id` - Unique plan identifier
+/// * `name` - Plan display name (e.g. "Starter", "Team", "Enterprise")
+/// * `price_eur` - Monthly price in euros
+/// * `features` - List of included features
+/// * `highlighted` - Whether this plan should be visually emphasized
+/// * `max_seats` - Maximum number of seats, None for unlimited
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct PricingPlan {
+    pub id: String,
+    pub name: String,
+    pub price_eur: u32,
+    pub features: Vec<String>,
+    pub highlighted: bool,
+    pub max_seats: Option<u32>,
+}
+
+/// Billing usage statistics for the current cycle.
+///
+/// # Fields
+///
+/// * `seats_used` - Number of active seats
+/// * `seats_total` - Total seats in the plan
+/// * `tokens_used` - Tokens consumed this billing cycle
+/// * `tokens_limit` - Token limit for the billing cycle
+/// * `billing_cycle_end` - ISO 8601 date when the current cycle ends
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct BillingUsage {
+    pub seats_used: u32,
+    pub seats_total: u32,
+    pub tokens_used: u64,
+    pub tokens_limit: u64,
+    pub billing_cycle_end: String,
+}