Fix Garmin connect race, surface tool error content, add dev start scripts

- GarminConnection now flushes Profile's pending debounced autosave before
  connecting, avoiding a race where Connect fires with stale credentials.
- client.go reads res.Content before checking IsError so tool error
  messages actually include mcp-garmin's response text.
- seedsample: update kind lookups to match current taxonomy names
  ("Easy Run" -> "Easy", "Interval" -> "Intervals").
- Add backend/start.sh and frontend/start.sh dev launch scripts, and
  check in CLAUDE.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/cmd/seedsample/main.go

@@ -43,9 +43,9 @@ func main() {
 	// gap between disjoint ranges. The taxonomy migration already seeded
 	// these rows (by fixed name) -- update their rules in place rather than
 	// creating new ones, since names are unique.
-	easyID := mustFindKindID(ctx, db, "Easy Run")
+	easyID := mustFindKindID(ctx, db, "Easy")
 	must(db.UpdateWorkoutKind(ctx, store.WorkoutKind{
-		ID: easyID, Name: "Easy Run", Description: "Easy conversational runs", Color: "#22c55e",
+		ID: easyID, Name: "Easy", Description: "Easy conversational runs", Color: "#22c55e",
 		RuleJSON: `{"match":"all","conditions":[
 			{"metric":"avg_pace_sec_per_km","op":"between","value":[330,420]},
 			{"metric":"avg_hr_pct_max","op":"<=","value":0.85}
@@ -63,9 +63,9 @@ func main() {
 		IsActive: true,
 	}))
 
-	intervalID := mustFindKindID(ctx, db, "Interval")
+	intervalID := mustFindKindID(ctx, db, "Intervals")
 	must(db.UpdateWorkoutKind(ctx, store.WorkoutKind{
-		ID: intervalID, Name: "Interval", Description: "Structured work/rest intervals", Color: "#ef4444",
+		ID: intervalID, Name: "Intervals", Description: "Structured work/rest intervals", Color: "#ef4444",
 		RuleJSON: `{"match":"all","conditions":[{"metric":"lap_interval_pattern","op":"==","value":true}]}`,
 		IsActive: true,
 	}))