[feature] self update (gitui update)

[shuppel]

This Pull Request closes any self-update-cli-command issue.

It changes the following:

• Adds gitui update subcommand for self-updating via CLI (before opening TUI)
• Supports updating via multiple package managers: cargo, dnf, apt, pacman, homebrew, scoop, chocolatey
• Detects installation method automatically by examining binary path and querying system package managers
• Supports --nightly / -n flag to include pre-release versions (nightly, rc, beta)
• Filters stable vs pre-release versions so users can choose update stability
• Adds short alias gitui -U for quick access to update command

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

Testing Performed

Tested on Fedora 42 with DNF-installed gitui:

• ./target/release/gitui update - Detects DNF, checks versions, prompts for update
• ./target/release/gitui update -n - Includes pre-release versions in update check
• ./target/release/gitui -U - Short alias works correctly

Module Structure

Created modular update system in src/update/:

• mod.rs - Orchestrates update flow, version checking, user interaction
• detector.rs - Installation method detection
• commands.rs - Update command execution using macro

Example Usage

# Update to latest stable
$ gitui update
gitui version: 0.28.1-nightly
Installation method: dnf
Update available: 0.28.1-nightly -> 0.28.1
Do you want to update gitui? [y/N]: y
Successfully updated via dnf!
# Update including pre-releases
$ gitui update -n
⚠️  Pre-release update available: 0.28.1-nightly -> 0.28.1-rc.1