Migrate CustomSteamLauncher to SteamFlow and add Ubuntu 24.04 .deb CI

[weter11]

Motivation

• Replace the old .NET CI and legacy project scaffold with a Rust-first launcher packaging workflow and name the crate consistently for packaging and docs.
• Move the login/session scaffold to a steam-vent 0.4.2-compatible refresh-token exchange flow so persistent refresh tokens can be reused and refreshed.
• Provide a CI path to produce distributable Debian packages on ubuntu-24.04 for the Rust launcher.

Description

• Removed the legacy .github/workflows/dotnet.yaml and added .github/workflows/rust.yaml which builds SteamFlow on ubuntu-24.04, installs native deps, runs cargo build --release, and produces a .deb via cargo-deb.
• Renamed the Rust project from custom-steam-launcher to steamflow (folder, crate name, docs and UI title) and updated paths and docs to SteamFlow.
• Updated SteamFlow/Cargo.toml with Debian metadata under [package.metadata.deb] so cargo-deb produces a steamflow package binary install target.
• Reworked session/auth logic in SteamFlow/src/steam_client.rs to: perform credential login (with optional 2FA), extract a refresh token from the initial response, exchange the refresh token via a subsequent CSourceUser_Logon_Request, and persist account_name+refresh_token in the session; added exchange_refresh_token and build_session helpers.
• Adjusted session model and config paths in SteamFlow/src/models.rs and SteamFlow/src/config.rs to include account_name and use ~/.config/SteamFlow/session.json and ./config/SteamFlow/images for caches.
• Replaced main bootstrap with an authenticate helper in SteamFlow/src/main.rs that attempts restore_session() and falls back to interactive login() when needed.
• Updated README and ROADMAP to reflect the SteamFlow name, new auth/token flow, and the new .github/workflows/rust.yaml packaging CI, and changed the UI empty-state heading to SteamFlow.

Testing

• Ran cargo fmt --manifest-path SteamFlow/Cargo.toml which completed successfully.
• Attempted cargo fetch / cargo check --manifest-path SteamFlow/Cargo.toml but dependency resolution failed due to network access to crates.io being blocked (CONNECT tunnel 403), so dependency compilation could not be validated in this environment.
• No other automated tests were present or executed as part of this change.

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Codex Task