Startup leaves no active profile when "default" is gone and the active profile was deleted: the #189 fix reopens #174 #196

What's wrong

PR #190 (d77c244) fixed #189 by having KeybindingManager.CreateDefaultProfile return early whenever any profile already exists (Keybinding/KeybindingManager.cs:228-231):

if (Profiles.GetAllProfiles().Count > 0)
{
    return null;
}

That return comes before any activation. The only activation path left runs when the profileId profile itself exists (lines 218-226).

So consider this state: "default" has been deleted, other profiles remain, and none of them is active. In that state the documented startup call returns null and leaves no profile active. That is the #174 symptom again, reached by a different path.

There are several ways to end up with no active profile:

• ProfileManager.DeleteProfile clears the active id when the active profile is deleted (Keybinding/Services/ProfileManager.cs:69-72).
• InitializeAsync leaves nothing active when active-profile.json is missing or blank.
• InitializeAsync also leaves nothing active when that file names an unknown id.

The XML doc for activation promises the opposite: "a startup call recovers from a deleted or unset active profile without overriding the one the user chose".

Reproduction (main @ 6d9183d, net10.0)

using (var m = new KeybindingManager(dir)) {
    await m.InitializeAsync(); m.CreateDefaultProfile();
    m.Profiles.CreateProfile("vim", "Vim"); m.Profiles.CreateProfile("emacs", "Emacs");
    m.Profiles.SetActiveProfile("vim");
    m.Profiles.DeleteProfile("default");
    m.Profiles.DeleteProfile("vim");          // active cleared; "emacs" remains
    await m.SaveAsync();
}
using (var m = new KeybindingManager(dir)) {  // next launch, README startup sequence
    await m.InitializeAsync();
    var r = m.CreateDefaultProfile();
    m.RegisterCommands([new Command("file.save", "Save")]);
    m.Keybindings.BindChord("file.save", Chord.Parse("Ctrl+S"));
}

Observed: returned=null active=none profiles=[emacs] bind=False.

The same happens on every later launch. BindChord returns false, ExecuteChord returns null, and nothing recovers on its own.

Suggested fix / acceptance criteria

• When profiles exist but none is active, CreateDefaultProfile(..., ProfileActivation.Activate) activates one deterministically and does not recreate default. One possible rule: the profileId profile if it exists, otherwise the only profile, otherwise the first by ordinal id. Document whichever rule is chosen in the method's XML docs.
• The Calling CreateDefaultProfile() at startup, as documented, recreates a deleted "default" profile and makes it the active profile on every launch #189 guarantees still hold: a deleted default is not recreated while other profiles exist, and an active profile is never replaced.
• A regression test covers the two-launch sequence above, next to ManagerStartupTests / DefaultProfileReactivationTests. On launch 2 a profile is active and BindChord succeeds.