refactor(store): single account name on users; rename profile/laps tables

users.display_name becomes users.name and is now the only human-facing
name -- profiles.name is dropped (it duplicated the account name; the
Profile page's Name field now edits users.name through PUT /api/profile,
which carries Name alongside the profiles columns). The profile table
becomes profiles and laps becomes activity_laps, homogeneous with
activity_samples. Onboarding pre-fills the display name from the OIDC
claim's name.

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

backend/internal/store/isolation_test.go

@@ -188,12 +188,12 @@ func TestIsolation_DeleteUserLeavesOtherUsersDataIntact(t *testing.T) {
 	if _, found, err := db.GetUserBySub(ctx, "sub-b"); err != nil || !found {
 		t.Fatalf("expected userB to survive userA's deletion, found=%v err=%v", found, err)
 	}
-	profileB, err := db.GetProfile(ctx, userB)
-	if err != nil {
+	if _, err := db.GetProfile(ctx, userB); err != nil {
 		t.Fatalf("GetProfile(b) after deleting a: %v", err)
 	}
-	if profileB.Name != "B" {
-		t.Errorf("userB's profile changed after deleting userA: %+v", profileB)
+	userBRow, found, err := db.GetUserBySub(ctx, "sub-b")
+	if err != nil || !found || userBRow.Name != "B" {
+		t.Errorf("userB's account changed after deleting userA: %+v (found=%v err=%v)", userBRow, found, err)
 	}
 	kindsB, err := db.ListWorkoutKinds(ctx, userB, false)
 	if err != nil || len(kindsB) != 8 {