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/AlternativeModal.jsx

@@ -0,0 +1,51 @@
+import { Icon } from './Icons'
+
+function AlternativeModal({ exercise, alternatives, loading, error, onSelect, onClose }) {
+  if (!exercise) return null
+
+  return (
+    <div className="alternative-modal-overlay" onClick={onClose}>
+      <div className="alternative-modal" onClick={(event) => event.stopPropagation()}>
+        <div className="alternative-modal-header">
+          <div>
+            <h3>Alternativa övningar</h3>
+            <p>För {exercise.name}</p>
+          </div>
+          <button className="alternative-modal-close" onClick={onClose} aria-label="Stäng">
+            <Icon name="chevronDown" size={18} />
+          </button>
+        </div>
+
+        {loading && (
+          <div className="alternative-modal-state">Laddar alternativ...</div>
+        )}
+
+        {!loading && error && (
+          <div className="alternative-modal-state error">{error}</div>
+        )}
+
+        {!loading && !error && alternatives.length === 0 && (
+          <div className="alternative-modal-state">Inga alternativ hittades.</div>
+        )}
+
+        {!loading && !error && alternatives.length > 0 && (
+          <div className="alternative-list">
+            {alternatives.map((alt) => (
+              <div key={alt.id} className="alternative-item">
+                <div className="alternative-info">
+                  <strong>{alt.name}</strong>
+                  <span>{alt.description || 'Ingen beskrivning tillgänglig.'}</span>
+                </div>
+                <button className="alternative-select-btn" onClick={() => onSelect(alt)}>
+                  Välj
+                </button>
+              </div>
+            ))}
+          </div>
+        )}
+      </div>
+    </div>
+  )
+}
+
+export default AlternativeModal