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/api/user.go

@@ -14,8 +14,8 @@ const resolvedUserContextKey userContextKey = iota
 // resolvedUser is the geniusrun account (if any) bound to the current
 // session's OIDC subject.
 type resolvedUser struct {
-	ID          int64
-	DisplayName string
+	ID   int64
+	Name string
 }
 
 // resolveUser runs after auth.RequireSession on every request and looks up
@@ -40,7 +40,7 @@ func (s *Server) resolveUser(next http.Handler) http.Handler {
 		}
 		ctx := r.Context()
 		if found {
-			ctx = context.WithValue(ctx, resolvedUserContextKey, resolvedUser{ID: u.ID, DisplayName: u.DisplayName})
+			ctx = context.WithValue(ctx, resolvedUserContextKey, resolvedUser{ID: u.ID, Name: u.Name})
 		}
 		next.ServeHTTP(w, r.WithContext(ctx))
 	})