fix: card-level buttons use ghost/outline instead of solid primary

Repeated per-card action buttons (Start Session, Resume, Save, Insert,
Start batch) now use cyan outline style to reduce visual noise.
Page-level and modal primary actions remain solid cyan.

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

frontend/src/components/library/TreeTableView.tsx

@@ -270,8 +270,8 @@ export function TreeTableView({
                     type="button"
                     onClick={() => onStartSession(tree.id, tree.tree_type)}
                     className={cn(
-                      'rounded-md bg-primary px-3 py-1.5 text-xs font-medium text-white',
-                      'hover:brightness-110 whitespace-nowrap'
+                      'rounded-md border border-primary/40 px-3 py-1.5 text-xs font-medium text-primary',
+                      'hover:bg-primary/10 hover:border-primary/60 transition-colors whitespace-nowrap'
                     )}
                   >
                     Start