feat(lists): allow nested sidebar folders via drag and drop

Folders can now contain other folders, to arbitrary depth, via a
self-referencing parent_folder_id (ON DELETE SET NULL, same pattern as
lists.folder_id). Nesting/un-nesting is drag-and-drop only: dropping a
folder onto another folder's header middle moves it in, the outer
quarters or any top-level/in-folder drop insert it beside a target.
Dropping a folder into itself or one of its own subfolders is silently
ignored. Deleting a folder releases its lists and subfolders back to
the top level, unchanged from the existing list behavior.

ListFolderNavItemViewModel.Lists became Children (object, mixing lists
and subfolders, mirroring TopLevel) so the existing implicit, type-matched
DataTemplates render nested folders recursively without new XAML, and each
nesting level gets its indent for free from the folder template's own
margin. Folders/RootLists/ContainerOf/PersistOrderAsync/RecomputeFolderCounts
now walk the tree instead of assuming one level.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/ClaudeDo.Ui/CLAUDE.md

@@ -41,7 +41,7 @@ Design/         — Tokens.axaml (design tokens; merged before styles)
 
 - **IslandsShellViewModel** — root coordinator. Owns the three island VMs and the `WorkerClient`, wires cross-island events (selection, notes/prep mode, conflict resolution), connection state, the update banner, the inline worker-log strip (clickable → Log Visualizer overlay; `FlashFooterError` surfaces UI-action failures + the worker's Warn/Error there), responsive-layout flags (`ShowLists`/`ShowDetails` by window width), `PrimeStatus` flash, the modal openers, and `RestartWorkerAsync`/`CheckForUpdatesAsync`. Hosts `UpdateCheckService`.
 - **ListsIslandViewModel** — smart lists (My Day, Important, Planned, virtual queued/running/review), user lists, selection, list CRUD, drag-reorder, badge counts, opens list settings / repo import / worktrees overview, `OpenInExplorer`/`OpenInTerminal`.
-  - **Folders:** the sidebar's top level is **one mixed collection**, `TopLevel` (`ObservableCollection<object>`: `ListFolderNavItemViewModel` rows, each with its own `Lists`, interleaved with folder-less `ListNavItemViewModel` rows) — that's what lets a list sit *above* a folder. `Folders`/`RootLists` are `OfType` **projections** of it, not collections to keep in sync. `UserLists` stays the **flat superset** every count refresh, row refresh and the no-repo banner work over — a row lives in both. `ContainerOf(row)` resolves which collection renders it (non-generic `IList`, since the top level is mixed). The view is a single `ItemsControl` over `TopLevel` with **implicit, type-matched `DataTemplate`s in `UserControl.DataTemplates`** — no keyed row template, and the folder's child `ItemsControl` resolves the same list template by walking up to the UserControl. A folder is never selectable: tapping the header runs `ToggleFolderCommand` (persisted to `list_folders.is_collapsed`), and its badge is the sum of its children. `DropAsync(source, target, placeBelow)` handles all four source/target combinations and doubles as the move: a drop position implies a container, so dropping a list onto a row in another folder — or next to *anything* at the top level, folder headers included — reassigns `FolderId`. A folder dropped onto a row inside a folder is ignored; folders never nest. `MoveToFolderAsync` is the drop-into-the-header path (reaches collapsed/empty folders, bound to the middle half of the header — the outer quarters insert *next to* the folder instead); `RemoveFromFolderCommand` is its context-menu counterpart. Both end in `PersistOrderAsync` → `ListRepository.ReorderSidebarAsync`, one `sort_order` sequence spanning **both tables**. ⚠️ A **folder** drag starts from `PointerMoved` past a 4px threshold using the *stored* `PointerPressedEventArgs` (`DoDragDropAsync` only accepts those) — starting it on press would capture the pointer and swallow the `Tapped` that folds the folder.
+  - **Folders:** the sidebar's top level is **one mixed collection**, `TopLevel` (`ObservableCollection<object>`: `ListFolderNavItemViewModel` rows, each with its own `Children`, interleaved with folder-less `ListNavItemViewModel` rows) — that's what lets a list sit *above* a folder. Folders nest arbitrarily deep — a folder's `Children` mixes lists and subfolders the same way `TopLevel` mixes folders and folder-less lists, via `ParentFolderId`. `Folders` is a **recursive** `OfType` projection over the whole tree (not just the top level); `RootLists` stays a `TopLevel`-only projection. `UserLists` stays the **flat superset** every count refresh, row refresh and the no-repo banner work over — a row lives in both. `ContainerOf(row-or-folder)` resolves which collection renders it (non-generic `IList`, since every container is mixed). The view is a single `ItemsControl` over `TopLevel` with **implicit, type-matched `DataTemplate`s in `UserControl.DataTemplates`** — no keyed row template, and the folder's child `ItemsControl` resolves the same templates recursively by walking up to the UserControl (which is also what indents nested folders: each level adds its template's fixed margin). A folder is never selectable: tapping the header runs `ToggleFolderCommand` (persisted to `list_folders.is_collapsed`), and its badge is the sum of all descendant lists, recursive. `DropAsync(source, target, placeBelow)` handles list and folder sources against any target and doubles as the move: a drop position implies a container, so dropping a row onto a row in another folder — or next to *anything* at the top level, folder headers included — reassigns `FolderId`/`ParentFolderId`. A folder dropped onto or next to a target inside its own subtree is silently ignored (self/descendant guard), never nested into itself. `MoveToFolderAsync` is the drop-into-the-header path (reaches collapsed/empty folders, bound to the middle half of the header — the outer quarters insert *next to* the folder instead; also takes a folder source now); `RemoveFromFolderCommand` is its context-menu counterpart. Both end in `PersistOrderAsync` → `ListRepository.ReorderSidebarAsync`/`SetFolderParentAsync`, one `sort_order` sequence spanning **both tables**, walked depth-first. ⚠️ A **folder** drag starts from `PointerMoved` past a 4px threshold using the *stored* `PointerPressedEventArgs` (`DoDragDropAsync` only accepts those) — starting it on press would capture the pointer and swallow the `Tapped` that folds the folder.
 - **TasksIslandViewModel** — open/overdue/completed groups for the selected list with hierarchy-aware regrouping; task CRUD, drag-reorder, toggle done/star, schedule, enqueue/dequeue, cancel; review actions; planning session lifecycle; `RefineTask`, `OpenConPtySessionRequested`, `ToggleManual`, `SyncInteractiveSessions`; MyDay extras (`IsMyDayList`, `ClearDayCommand`, `ShowPrepLogCommand`) and the pinned Notes pseudo-row. Raises `NotesRequested`/`PrepRequested` for the shell.
 - **DetailsIslandViewModel** — the detail pane for a bound `TaskRowViewModel`. Owns live-log streaming (`Log` via `StreamLineFormatter`), debounced title/description editing, subtasks, the session-outcome/roadblock split, the three-tab work console (`output`/`git`/`session`), child surfacing (`ChildOutcomes` + `ChildrenNeedingAttention` drive an attention band on the Session tab), attachments, and the modes `IsNotesMode`/`IsPrepMode`/computed `IsTaskDetailVisible`. Failures raise `ErrorReported`, wired by the shell into `FlashFooterError`.
   - Three concerns are extracted into section VMs exposed as properties: `AgentSettings` (`AgentConfigEditorViewModel`, scope=Task), `MergeSectionViewModel`, `PrepPanelViewModel`. Helper rows live in the same file.