feat: replace per-workout-type phase-detection settings with one global pair

The 14 phase-related profile columns (warmup/cooldown per workout type,
plus an unused Interval pair) added more granularity than wanted. Migration
0006 drops all 14 and adds a single warmup_minutes/cooldown_minutes pair
applied uniformly to every fixed-duration workout type; Interval still
detects phases from lap data directly and ignores this setting. Restores
a simplified Phase detection section in the Profile UI with just the two
fields.

frontend/src/pages/Profile.tsx

@@ -146,6 +146,25 @@ export function Profile() {
         ))}
       </fieldset>
 
+      <fieldset className="kind-editor">
+        <legend>Phase detection</legend>
+        <div className="controls">
+          <NumberField
+            label="Warm-up (minutes)"
+            value={profile.WarmupMinutes}
+            onChange={(v) => set("WarmupMinutes", v)}
+          />
+          <NumberField
+            label="Cool-down (minutes)"
+            value={profile.CooldownMinutes}
+            onChange={(v) => set("CooldownMinutes", v)}
+          />
+        </div>
+        <p className="empty-state">
+          Applies to every workout type. Interval workouts detect warm-up/cool-down from lap data directly and don't use this setting.
+        </p>
+      </fieldset>
+
       <button onClick={save}>Save</button>
     </div>
   );