design: WorkoutPage Hevy-style redesign + AlternativeModal + backend API

- Add GET /api/exercises/:id/alternatives endpoint
- Add GET /api/exercises/:id/last-workout endpoint
- New AlternativeModal component for swapping exercises
- WorkoutPage: single-tap logging, +/- buttons, rest timer
- Updated Icons with new workout icons
- Polish: card shadows, borders, micro-interactions
- Tasks directory for project management

frontend/src/components/Icons.jsx

@@ -62,6 +62,14 @@ export const Icons = {
       <line x1="5" y1="12" x2="19" y2="12"/>
     </svg>
   ),
+  swap: (
+    <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+      <polyline points="7 7 3 11 7 15"/>
+      <polyline points="17 9 21 13 17 17"/>
+      <line x1="3" y1="11" x2="21" y2="11"/>
+      <line x1="3" y1="13" x2="21" y2="13"/>
+    </svg>
+  ),
   check: (
     <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2.5" strokeLinecap="round" strokeLinejoin="round">
       <polyline points="20 6 9 17 4 12"/>