refactor: replace SVG sprite with client-side icon fetching and Rust caching

Eliminates server-side batch rendering and IconSprite pattern in favor of:
- Rust handler with moka cache (10k icons, 24h TTL, immutable HTTP headers)
- Client Icon component fetches SVGs on-demand with shimmer loading states
- Removes renderIconsBatch, tag-icons collection logic, and page load icon preprocessing
- Reduces SSR complexity and data serialization overhead

src/state.rs

@@ -2,7 +2,7 @@ use std::sync::Arc;
 
 use crate::{
     auth::SessionManager, cache::IsrCache, health::HealthChecker, http::HttpClient,
-    tarpit::TarpitState,
+    icon_cache::IconCache, tarpit::TarpitState,
 };
 
 /// Application state shared across all handlers
@@ -14,6 +14,7 @@ pub struct AppState {
     pub pool: sqlx::PgPool,
     pub session_manager: Arc<SessionManager>,
     pub isr_cache: Arc<IsrCache>,
+    pub icon_cache: Arc<IconCache>,
 }
 
 /// Errors that can occur during proxying to Bun