Fix browser crash: cache schema context, lazy Dashboard, default to Ask tab

Root cause: Dashboard auto-fired 6+ API calls on load, then Ask tab fired
7 DESCRIBE queries per question — 15+ concurrent requests from WASM.

Fixes:
- Schema context cached after first build (7 DESCRIBE → 0 on subsequent questions)
- Dashboard lazy-loads only when tab clicked (not on app mount)
- Default tab changed back to Ask (no background API storm)
- std::sync::Mutex for WASM compat (no tokio in browser)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/ui/src/main.rs

@@ -109,8 +109,30 @@ async fn fetch_health(path: &str) -> Result<String, String> {
     resp.text().await.map_err(|e| e.to_string())
 }
 
+/// Cached schema context — built once, reused across questions.
+static SCHEMA_CACHE: std::sync::OnceLock<std::sync::Mutex<Option<String>>> = std::sync::OnceLock::new();
+
+async fn get_schema_context_cached(datasets: &[Dataset]) -> String {
+    // Check cache first
+    {
+        let cache = SCHEMA_CACHE.get_or_init(|| std::sync::Mutex::new(None));
+        if let Ok(guard) = cache.lock() {
+            if let Some(ref cached) = *guard {
+                return cached.clone();
+            }
+        }
+    }
+    // Build and cache
+    let ctx = get_schema_context(datasets).await;
+    if let Some(cache) = SCHEMA_CACHE.get() {
+        if let Ok(mut guard) = cache.lock() {
+            *guard = Some(ctx.clone());
+        }
+    }
+    ctx
+}
+
 /// Get schema context for datasets (used for AI SQL generation).
-/// Limits to core tables to keep prompt size reasonable.
 async fn get_schema_context(datasets: &[Dataset]) -> String {
     let core_tables = ["candidates", "clients", "job_orders", "placements", "timesheets", "call_log", "email_log"];
     let mut ctx = String::from("DATABASE SCHEMA:\n\n");
@@ -163,7 +185,7 @@ enum Tab {
 
 #[component]
 fn App() -> Element {
-    let mut active_tab = use_signal(|| Tab::Dashboard);
+    let mut active_tab = use_signal(|| Tab::Ask);
     let mut datasets = use_signal(Vec::<Dataset>::new);
     let mut ds_loading = use_signal(|| true);
 
@@ -268,7 +290,7 @@ fn AskPanel(datasets: Vec<Dataset>) -> Element {
 
             // Step 1: Get schema context
             step.set("reading schemas...".into());
-            let schema_ctx = get_schema_context(&ds).await;
+            let schema_ctx = get_schema_context_cached(&ds).await;
 
             // Step 2: Generate SQL
             step.set("writing SQL...".into());
@@ -351,7 +373,7 @@ fn AskPanel(datasets: Vec<Dataset>) -> Element {
                                 generated_sql.set(None);
                                 result.set(None);
                                 step.set("reading schemas...".into());
-                                let schema_ctx = get_schema_context(&ds).await;
+                                let schema_ctx = get_schema_context_cached(&ds).await;
                                 step.set("writing SQL...".into());
                                 let prompt = format!(
                                     "You are a SQL assistant. You write Apache DataFusion SQL (PostgreSQL-compatible).\n\n\
@@ -673,8 +695,14 @@ fn DashboardPanel() -> Element {
     };
 
     // Auto-load on mount
-    // Auto-load on mount
-    use_effect(move || { do_load(); });
+    // Load on first render (user clicks Dashboard tab)
+    let mut loaded = use_signal(|| false);
+    use_effect(move || {
+        if !*loaded.read() {
+            loaded.set(true);
+            do_load();
+        }
+    });
 
     rsx! {
         div { class: "panel",