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/styles.css

@@ -2500,6 +2500,25 @@ a.nav-item,
   color: var(--muted);
 }
 
+.session-card {
+  position: relative;
+}
+
+.session-card.busy {
+  opacity: 0.7;
+}
+
+.session-busy-overlay {
+  position: absolute;
+  inset: 0;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: rgba(var(--bg-rgb, 255, 255, 255), 0.8);
+  border-radius: var(--space-2);
+  z-index: 1;
+}
+
 /* ============================================
    Terminal Styles
    ============================================ */