fix: JoinPage graceful already-member handling + "I use another instance" for auth'd users

- Add spaceId to InvitePreview so the client can navigate to the space
- Detect "already a member" error and show green success card with auto-redirect
- Add "I use another instance" link for authenticated users alongside "Not you? Log in"

packages/server/src/routes/spaces.ts

@@ -1103,6 +1103,7 @@ export async function spaceRoutes(app: FastifyInstance): Promise<void> {
     const instanceName = settings?.instanceName ?? 'Backspace';
 
     return reply.code(200).send({
+      spaceId: space.id,
       spaceName: space.name,
       description: space.description ?? null,
       icon: space.icon ?? null,