[Bug] Remote name with uppercase letters is forced to lowercase in GUI (e.g., 'G' -> 'g')

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Description

Remote names containing uppercase letters (e.g. G, Origin, MyRemote) are displayed as all-lowercase in the UI (e.g. g, origin, myremote).

Root Cause

1. In commit 582c1a2217001d1fae0c1f89e80d9a62407be007 (refactor: repository remote), Commands.QueryRemotes was changed to extract remote names directly from Commands.Config.ReadAllAsync() instead of parsing the output of git remote -v.
2. In Commands.Config.ReadAllAsync(), the config key is lowercased unconditionally:

   var key = parts[0].ToLower(CultureInfo.CurrentCulture);

3. According to the Git configuration specification:
   • Section (e.g., remote) and Variable name (e.g., url) are case-insensitive.
   • Subsection (e.g., <name> in remote.<name>.url) is case-sensitive.
     Lowercasing the full key converts remote.G.url to remote.g.url, causing QueryRemotes to extract the remote name as g rather than G.

Steps to Reproduce

1. In a repository, configure a remote with uppercase letters, e.g.:

   git remote add G git@github.com:example/repo.git

2. Open SourceGit and inspect the repository.
3. Observe the remote name in the sidebar remotes tree, fetch/pull/push dialogs, and commit decorators.
4. It is shown as g instead of G.

Suggested Solution

When canonicalizing git config keys, preserve the case of subsections (between the first and last dots):

public static string CanonicalizeKey(string key)
{
    if (string.IsNullOrEmpty(key))
        return key;

    var firstDot = key.IndexOf('.');
    if (firstDot < 0)
        return key.ToLower(CultureInfo.CurrentCulture);

    var lastDot = key.LastIndexOf('.');
    if (firstDot == lastDot)
        return key.ToLower(CultureInfo.CurrentCulture);

    var section = key.Substring(0, firstDot).ToLower(CultureInfo.CurrentCulture);
    var subsection = key.Substring(firstDot, lastDot - firstDot);
    var variable = key.Substring(lastDot).ToLower(CultureInfo.CurrentCulture);
    return string.Concat(section, subsection, variable);
}

And apply it when parsing keys in Config.ReadAll() and Config.ReadAllAsync().