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.Localization/locales/en.json

@@ -805,7 +805,7 @@
     "sessionSkillsTab": { "installed": "Installed: {0}", "installFailed": "Install failed: {0}", "updated": "Updated.", "updateFailed": "Update failed: {0}", "removed": "Removed.", "removeFailed": "Remove failed: {0}" },
     "worktreesTab": { "workerOffline": "Worker offline.", "removed": "Removed {0} worktree(s).", "blocked": "Cannot force-remove: {0} task(s) still running. Cancel them first.", "removedFrom": "Removed {0} worktree(s) from {1} task(s).", "cleanupFailed": "Cleanup failed: {0}", "resetFailed": "Reset failed: {0}" },
     "worktreesOverview": { "titleAll": "Worktrees", "titleList": "Worktrees — {0}", "listFallback": "list", "cleanupFailed": "Cleanup failed.", "cleanupFailedDetailed": "Cleanup failed: {0}", "removed": "Removed {0} worktree(s).", "discardConfirm": "Discard the worktree for \"{0}\"? Uncommitted work will be lost.", "discardFailed": "Failed to discard worktree.", "keepFailed": "Failed to keep worktree.", "cannotForceRunning": "Cannot force-remove a running task.", "forceRemoveFailed": "Force remove failed.", "forceRemoveFailedDetailed": "Force remove failed: {0}", "batchDone": "Merged {0}, {1} need resolution.", "openInExplorerFailed": "Couldn't open path: {0}" },
-    "listSettings": { "untitled": "Untitled", "ticketProjectNone": "— none —", "loadFailed": "Couldn't load list settings: {0}", "ticketProjectsLoadFailed": "Couldn't load ticket projects: {0}", "confirmDeleteFolder": "Delete folder \"{0}\"? The lists inside it are kept and move back to the top level." },
+    "listSettings": { "untitled": "Untitled", "ticketProjectNone": "— none —", "loadFailed": "Couldn't load list settings: {0}", "ticketProjectsLoadFailed": "Couldn't load ticket projects: {0}", "confirmDeleteFolder": "Delete folder \"{0}\"? The lists inside it are kept and move back to the top level.", "confirmDeleteList": "Delete list \"{0}\" and all its tasks? This cannot be undone.", "deleteBlockedPlanning": "This list has planning sessions with child tasks. Discard those first, then delete the list.", "workingDirMissing": "This folder doesn't exist.", "workingDirNotRepo": "This folder isn't a git repository yet. Run `git init` in it first, or clear the working directory to save this as a manual list." },
     "detailsIsland": { "verifyFailed": "Merge landed, but the list's verify command failed — the task was kept out of Done.", "untrackedCollision": "Merge refused — it would overwrite an untracked file in the target working directory.", "stopOffline": "Worker offline — can't stop the task.", "stopFailed": "Stop failed: {0}", "enqueueFailed": "Queue failed: {0}", "dequeueFailed": "Remove from queue failed: {0}", "resetAndRetryFailed": "Reset & retry failed: {0}", "toggleDoneFailed": "Couldn't update done state: {0}" },
     "mergeSection": { "openWorktreeFailed": "Couldn't open worktree: {0}" },
     "about": { "openPathFailed": "Couldn't open path: {0}" },