fix(ui): warn and block save when a list's working dir isn't a git repo

Reuses RepoScanner.IsRepo (now public) to validate WorkingDir whenever it
changes in ListSettingsModalViewModel — Browse or typing. A missing folder
or a non-repo folder shows an inline warning and gates Save via the same
CanExecute pattern the load-failure gate already uses; an empty WorkingDir
is untouched. Previously this only surfaced as a Failed task deep in
WorktreeManager on first run.

Also localizes the two hardcoded EN strings in the same file (delete
confirm, planning-sessions block) into vm.listSettings.* keys.

src/ClaudeDo.Ui/Services/RepoScanner.cs

@@ -69,5 +69,5 @@ public static class RepoScanner
 
     // A real clone has .git as a directory. Worktrees and submodules have it as a file
     // ("gitdir: ...") — those are not standalone repos and would flood the list.
-    private static bool IsRepo(string dir) => Directory.Exists(Path.Combine(dir, ".git"));
+    public static bool IsRepo(string dir) => Directory.Exists(Path.Combine(dir, ".git"));
 }