refactor: Abstract display blocks with self-contained data fetching and rendering

[unhappychoice]

Summary

Refactor the codebase to encapsulate each display block so that it can declare required resources and render its own output.

Current State

• Data fetching is centralized in SteamClient
• Display rendering is handled separately in display.rs
• Adding new blocks requires changes in multiple places

Proposed Design

Each display block should:

• Declare what resources it needs (not fetch directly)
• Render its own output using fetched resources

// Example concept
enum Resource {
    Player,
    OwnedGames,
    Achievements,
    SteamLevel,
    RecentlyPlayed,
}

trait DisplayBlock {
    fn required_resources(&self) -> Vec<Resource>;
    fn render(&self, resources: &ResourceStore) -> Vec<String>;
}

// Fetcher collects all required resources from blocks,
// deduplicates, and fetches them once
struct ResourceFetcher { ... }

Benefits

• Shared resources (e.g., OwnedGames) fetched only once
• Easier to add/remove/reorder display blocks
• Better separation of concerns
• Each block can manage its own caching strategy
• More testable components

[linear]

E-778