feat(05-03): Exercise research frontend integration

- Add ExerciseResearchPanel component with Get Research button, loading state, summary display, and source links
- Add ExerciseEncyclopediaPage with exercise list and integrated research panel
- Wire encyclopedia view into App.jsx navigation
- Add encyclopedia nav button to Dashboard
- Add CSS for research panel and encyclopedia search

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/Dashboard.jsx

@@ -98,6 +98,7 @@ function Dashboard({ onStartWorkout, onNavigate }) {
           <nav className="nav-menu">
             <button className="nav-btn active"><Icon name="home" size={18} /></button>
             <button className="nav-btn" onClick={() => onNavigate('progress')}><Icon name="chart" size={18} /></button>
+            <button className="nav-btn" onClick={() => onNavigate('encyclopedia')} title="Exercise Encyclopedia"><Icon name="search" size={18} /></button>
             <button className="nav-btn" onClick={() => onNavigate('profile')}><Icon name="user" size={18} /></button>
             <button className="nav-btn logout" onClick={logout}><Icon name="logout" size={18} /></button>
           </nav>