refactor: rename Step Library → Solutions Library site-wide

User-facing text updated across page title, sidebar, command palette,
quick launch, quick actions, post-step modal, empty state, and
procedural editor. File paths/component names unchanged — those
will be renamed when the full Solutions Library feature ships.

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

frontend/src/components/session/PostStepActionModal.tsx

@@ -50,7 +50,7 @@ export function PostStepActionModal({
               <div>
                 <p className="font-medium text-foreground">Save for Later</p>
                 <p className="text-sm text-muted-foreground">
-                  Add to your step library for future use
+                  Save to solutions library for future use
                 </p>
               </div>
             </div>