Merge backfill into Sync now, replace Full backfill with a destructive Reset all

Sync now now runs Backfill (resumes from the watermark, so a widened history
horizon is picked up automatically) then IncrementalSync then detail-fill, in
one click. "Full backfill" is gone -- in its place, "Reset all" (styled as a
destructive action, gated by a confirm dialog) wipes every synced activity
and its laps/kind assignments and rewinds the backfill watermark, so the next
sync performs a genuinely fresh pull instead of trying to patch up existing
rows with newer schema fields.

backend/internal/store/reset_test.go

@@ -0,0 +1,71 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+func TestResetAllSyncedData_DeletesActivitiesCascadesAndRewindsWatermark(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+
+	activityID, err := db.UpsertActivity(ctx, Activity{GarminActivityID: 1, ActivityType: "running", StartTimeUTC: "2026-07-11 06:00:00", RawJSON: "{}"})
+	if err != nil {
+		t.Fatalf("UpsertActivity: %v", err)
+	}
+	kindID, err := db.CreateWorkoutKind(ctx, WorkoutKind{Name: "Test Reset Kind", RuleJSON: `{"match":"all","conditions":[]}`, IsActive: true})
+	if err != nil {
+		t.Fatalf("CreateWorkoutKind: %v", err)
+	}
+	if err := db.ReplaceLaps(ctx, activityID, []Lap{{LapIndex: 1}}); err != nil {
+		t.Fatalf("ReplaceLaps: %v", err)
+	}
+	if _, err := db.InsertKindAssignment(ctx, KindAssignment{
+		ActivityID: activityID, WorkoutKindID: &kindID, AssignmentSource: AssignmentSourceRuleEngine,
+		Status: AssignmentStatusAssigned, CandidateKindsJSON: "[]",
+	}); err != nil {
+		t.Fatalf("InsertKindAssignment: %v", err)
+	}
+	if err := db.UpdateSyncState(ctx, "2020-01-01", true); err != nil {
+		t.Fatalf("UpdateSyncState: %v", err)
+	}
+
+	if err := db.ResetAllSyncedData(ctx); err != nil {
+		t.Fatalf("ResetAllSyncedData: %v", err)
+	}
+
+	activities, err := db.ListActivities(ctx, ActivityFilter{})
+	if err != nil {
+		t.Fatalf("ListActivities: %v", err)
+	}
+	if len(activities) != 0 {
+		t.Errorf("expected 0 activities after reset, got %d", len(activities))
+	}
+	laps, err := db.LapsForActivity(ctx, activityID)
+	if err != nil {
+		t.Fatalf("LapsForActivity: %v", err)
+	}
+	if len(laps) != 0 {
+		t.Errorf("expected laps to cascade-delete, got %d", len(laps))
+	}
+	if _, ok, err := db.CurrentAssignment(ctx, activityID); err != nil || ok {
+		t.Errorf("expected kind assignment to cascade-delete, ok=%v err=%v", ok, err)
+	}
+
+	state, err := db.GetSyncState(ctx)
+	if err != nil {
+		t.Fatalf("GetSyncState: %v", err)
+	}
+	if state.EarliestSyncedDate != nil || state.BackfillComplete {
+		t.Errorf("expected watermark rewound to fresh state, got %+v", state)
+	}
+
+	// Workout kinds (the user's taxonomy) must survive a reset.
+	kind, ok, err := db.GetWorkoutKind(ctx, kindID)
+	if err != nil || !ok {
+		t.Fatalf("GetWorkoutKind after reset: ok=%v err=%v", ok, err)
+	}
+	if kind.Name != "Test Reset Kind" {
+		t.Errorf("workout kind was unexpectedly affected by reset: %+v", kind)
+	}
+}