fix: unify session loading states across pages

- Add per-item busy overlay to SessionCard component
- Remove full-screen loading overlay from sessions page
- Remove loadingAction state, use per-item busy state only
- Add handleStart to sessions page for consistency
- Add session-card CSS for busy overlay positioning
- Both home and sessions pages now use same per-item loading pattern

apps/web/src/components/session-card.tsx

@@ -75,7 +75,12 @@ export function SessionCard({
   const isActive = ["running", "building", "starting", "probing", "pending", "unhealthy"].includes(session.status);
 
   return (
-    <article className="card session-card">
+    <article className={`card session-card ${isBusy ? "busy" : ""}`}>
+      {isBusy && (
+        <div className="session-busy-overlay">
+          <Icon name="loading" size="md" />
+        </div>
+      )}
       <div className="session-card-content">
         <div className="session-card-header">
           <div className="session-card-title">