Fix installed-game detection and post-install install path handling

[weter11]

Motivation

• Steam libraries were not being detected properly because the default steam_library_path was a hard-coded nonstandard directory, causing installed games to always appear as not installed.
• The scanning fallback in scan_installed_app_paths_pathbuf() never reached the Steam-detection fallback when the configured path was invalid, so the code fell back to an unexpanded literal path.
• After an install completes, the UI stamped a temporary download path as the game's install_path instead of the real installation location, preventing the Play button from appearing.

Description

• Prefer an actual detected Steam install when constructing LauncherConfig::default() by calling detect_steam_path() and only fall back to ~/Games/SteamFlow if detection fails (file: SteamFlow/src/config.rs).
• Restructure scan_installed_app_paths_pathbuf() to use the configured path when valid, otherwise fall back to detect_steam_path() and then default_steam_root() (file: SteamFlow/src/library.rs).
• On download completion in poll_download_progress(), rescan installed app paths with scan_installed_app_paths() and set the library entry's install_path to the real path (and update is_installed accordingly), instead of assigning a $TMPDIR download path (file: SteamFlow/src/ui.rs).

Testing

• Ran cargo fmt --all in SteamFlow/ to format changes (succeeded).
• Ran cargo check in SteamFlow/ which completed successfully; build warnings unrelated to these changes remain but are non-blocking.

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Codex Task