fix: improve error message when git is not installed

[YuriNachos]

Summary

• When git is not installed at all, show a more helpful message instead of suggesting to upgrade git version
• Adds detection for "executable file not found" / "command not found" errors
• Shows "Git is not installed. Please install git to use lazygit." instead of "Git version must be at least 2.32.0. Please upgrade your git version."

Changes

• Added GitNotInstalledError translation key in pkg/i18n/english.go
• Added isGitNotInstalledError() helper function in pkg/app/app.go
• Updated validateGitVersion() to check error type and show appropriate message

Motivation

Fixes #5028 - the error message was confusing when git wasn't installed at all

Test manual

# Temporarily rename git to simulate "not installed"
sudo mv /usr/bin/git /usr/bin/git.bak
lazygit
# Expected: "Git is not installed. Please install git to use lazygit."

# Restore git
sudo mv /usr/bin/git.bak /usr/bin/git

Checklist

• Code compiles (go build)
• Code formatted with gofumpt
• Commit message follows conventional commits