backend: split tauri commands module

[Jordonbc]

Summary

• replace the monolithic tauri_commands.rs file with a directory-based module tree that groups commands by area
• extract shared helpers into shared.rs and re-export the command functions via mod.rs
• move existing command implementations into focused files for general repo management, branches, status/logging, stashes, commits, remotes, LFS, backends, settings, and updater handling

Testing

• cargo fmt
• cargo check (fails: missing system dependency glib-2.0 required by glib-sys)

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https://chatgpt.com/codex/tasks/task_e_68c992b48844832286f5accd6de90e1b