Add a friends hub with realtime online presence

[coder13]

Goal

Give authenticated users one dependable place to see friendships, pending requests, and which friends are currently available to race.

User experience

Add a /friends route reachable from the header/profile menu with:

• online friends first, then offline friends;
• incoming and outgoing pending requests;
• actions to open a profile, accept/decline/cancel a request, unfriend, block, or invite an eligible friend to the current room;
• clear empty, loading, disconnected, and error states;
• a responsive layout that works with the existing mobile navigation.

Presence contract

• Presence is ephemeral and friends-only in this surface.
• "Online" means the user has at least one authenticated Let’s Cube socket connected.
• Multiple tabs/devices count as one user; offline is emitted only after the last socket disconnects.
• Do not persist or display last seen in the MVP.
• Do not create durable "friend came online" notifications in the MVP.
• REST owns the durable friend list; Socket.IO only delivers presence changes. Reconnect must refetch/reconcile durable state.

Acceptance criteria

• Add friend-list/request state and selectors without coupling React components directly to sockets.
• Move friend presence through the global namespace and existing per-user Socket.IO rooms.
• Compute first-connect/last-disconnect correctly across multiple processes through the Redis adapter.
• Deliver an initial presence snapshot and incremental updates only for accepted friendships.
• Handle Redis/socket interruption without deleting friendships or permanently showing stale presence.
• Add focused server tests for multi-tab, multi-process, blocked/unfriended, and reconnect behavior.
• Add client tests for sorting, request states, offline/error states, responsive actions, and reconnect reconciliation.
• Add a Cypress happy path with two authenticated test users.

Dependencies

• Build the friend request, friendship, and blocking lifecycle #75 friendship lifecycle
• Add a typed in-app notification inbox for social actions #105 notification/request state
• Add privacy-safe cuber discovery and public profiles #82 public profile links

Non-goals

• Persistent last-seen timestamps.
• Browser/OS push notifications.
• Direct messaging.