feat: support multiple Steam accounts

[yanekyuk]

Implements #28.

Summary

• Adds SteamAccount type and read_steam_accounts() to parse loginusers.vdf, resolving SteamID64 → display name mappings
• Stores preferred_steam_account: Option<u64> (SteamID64) in config.toml; read_launch_options uses it to target the right userdata/ directory, falling back to the most-recently-modified heuristic
• First-run flow: prompts users with 2+ accounts to pick one; auto-selects silently if only one account exists
• Settings screen: new Steam Account field (field 3, cycle with ←/→) showing PersonaName (AccountName) or Auto (most recent)

Test Plan

• cargo test passes (60 tests)
• On a machine with multiple Steam accounts, first-run shows the account picker after the library setup step
• On a machine with one account, no picker appears and the account is auto-selected
• Settings screen shows the Steam Account field; ←/→ cycles accounts; Esc saves
• Launch options display uses the preferred account's localconfig.vdf
• Existing config.toml without preferred_steam_account deserializes cleanly (field defaults to None)