Add privacy-safe cuber discovery and public profiles

[coder13]

Goal

Help authenticated cubers find someone they already know or raced with and open a safe public profile before sending a friend request.

MVP discovery contract

• Search only by normalized username and, when the target has explicitly enabled WCA identity visibility, WCA ID.
• Never search by email, ingest email for this feature, accept email as an identifier, or reveal whether an email exists.
• Require authentication, enforce bounded/rate-limited queries, and return a capped paginated result set.
• Respect blocks and profile visibility without telling a blocked user that a block caused an omitted result.
• Support entry points from the lobby user list, room users/times/chat, the friends hub, and direct username search.
• Do not add algorithmic stranger recommendations in the MVP.

Public profile contract

Use one explicit server-side public projection containing only fields the viewer may see:

• stable public user identifier;
• display name and username;
• WCA identity and WCA-hosted avatar only when the target opted to reveal them;
• viewer-relative friend state: none, outgoing request, incoming request, or friends;
• relevant actions: request/cancel/accept/decline/unfriend/block and invite when eligible.

Do not reuse the editable /profile response for another user, and never include access tokens, email, hidden real name/WCA ID, private preferences, private-room membership, friend graph, or notification data.

Acceptance criteria

• Add authenticated, indexed user search backed by Normalize usernames for safe, indexed user lookup #185's normalized username field.
• Add an authenticated public-profile endpoint and /users/:id client route with explicit field projection.
• Make user names/avatars open the public profile from existing lobby and room surfaces without breaking timer interaction or mobile layouts.
• Show correct viewer-relative friendship actions using Build the friend request, friendship, and blocking lifecycle #75 and handle concurrent/stale transitions.
• Return stable empty, not-found/unavailable, blocked, rate-limited, loading, and error states without user-enumeration leaks.
• Add server tests for every visibility combination, email-like queries, blocks, pagination, ID tampering, and unauthenticated access.
• Add client tests for hidden/visible WCA identity, friend states, keyboard accessibility, and responsive layouts.
• Add a two-user Cypress flow for search → profile → friend request.

Dependencies

• Normalize usernames for safe, indexed user lookup #185 normalized username migration
• Stop requesting, storing, and mirroring WCA email addresses #191 email removal/purge
• Build the friend request, friendship, and blocking lifecycle #75 friendship lifecycle

Deferred

• Opt-in language/timezone/event/pace matching is tracked in Add opt-in racer preferences and discovery filters #190.
• Solve history, PBs, and rankings depend on the results-history work and are not required for this MVP.
• Public unauthenticated profiles are out of scope.