feat(ui): collapsible folders for the list sidebar

Folders render above the top-level lists, each with its own collapsible
children; tapping a folder header folds it instead of selecting it.
Lists move in by dragging onto a folder header (or onto a row inside it)
and out by dropping between top-level rows or via the context menu.

Creating a folder is a toggle in the New-list modal, not a third sidebar
button: it reduces the modal to name + icon and writes a folder row
instead of the list row the create flow pre-created.

src/ClaudeDo.Localization/locales/en.json

@@ -260,6 +260,8 @@
     "contextOpenFindings": "Open findings folder",
     "contextImportTickets": "Update from ticket system",
     "contextSettings": "Settings…",
+    "contextFolderSettings": "Folder settings…",
+    "contextRemoveFromFolder": "Move out of folder",
     "newList": "New list",
     "addReposTip": "Add repos as lists",
     "noRepoBannerText": "No repo linked yet. ClaudeDo runs tasks in a git repo.",
@@ -403,7 +405,11 @@
     },
     "listSettings": {
       "title": "LIST SETTINGS",
+      "folderTitle": "FOLDER",
       "deleteList": "Delete list",
+      "deleteFolder": "Delete folder",
+      "createAsFolder": "Create a folder instead",
+      "createAsFolderHint": "A folder just groups lists in the sidebar — it holds no tasks and runs nothing. Drag lists onto it to fill it.",
       "sectionGeneral": "GENERAL",
       "name": "Name",
       "workingDirectory": "Working directory",
@@ -793,7 +799,7 @@
     "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}" },
+    "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." },
     "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}" },