feat: resize todo panes and collapse note groups

Let users drag the to-do list/detail divider and remember its width. Note headings independently collapse task rows without clearing selected details. Use a 1px divider with a transparent drag target and compact spacing.

Validation: 115 focused tests including native Electron input; typecheck and lint passed.

Release note: Resize the Magic Notes to-do list with a thin draggable divider and collapse tasks by source note.

docs/features/magic-notes/README.md

@@ -15,7 +15,9 @@ above the history, with text/canvas selection and Save. Creating a record resets
 that composer; editing a saved record requires its explicit Edit action. The separate To-dos view
 keeps search, status filters and grouped tasks on one page, with direct completion
 and a stable left list/right detail layout for instructions, source content and AI
-comments. Narrow containers use on-demand detail with an explicit return to the
+comments. The divider can be dragged and remembers its width; each note group
+can independently collapse its tasks while retaining its title and count.
+Narrow containers use on-demand detail with an explicit return to the
 mounted list. One content-sized button switches to the other view in the page
 header immediately before New note; status filters sit beside search and wrap
 when needed. Note detail removes the surrounding frame and stream padding while

docs/features/magic-notes/progress.md

@@ -1,5 +1,18 @@
 # Magic Notes Progress
 
+## 2026-09-24: Resizable Todo List and Collapsible Note Groups
+
+- Todo list/detail uses a 1px divider with a transparent 9px pointer target and
+  8px content padding on each side. Dragging and keyboard resizing persist the
+  list width; narrow layouts retain the saved desktop preference.
+- Source-note headings independently collapse their task rows while retaining
+  the count and selected detail. Buttons support native keyboard activation.
+- Focused workspace, disclosure and Electron layout tests passed: 3 files,
+  115 tests. Native Electron input verified dragging, keyboard expansion,
+  width restoration after reload and narrow-window transitions. Typecheck and
+  lint passed. A full-suite validation attempt was cancelled; no full-suite
+  success is claimed for this change.
+
 ## 2026-09-20: Shared Inline Help
 
 The canvas page-count question mark now uses `InlineHelp`, superseding the native

docs/features/magic-notes/technical-design.md

@@ -392,8 +392,16 @@ At a page container width of 700px or less, CSS hides the mounted list while a
 task is selected and shows an explicit return button in the detail area. Returning
 clears selection and restores row focus without scrolling; source-note return
 retains selection and focuses this visible return button on narrow layouts.
-There is no todo detail route, separate AI sidebar or resize handle. Note detail
-retains its existing AI pane preferences.
+There is no todo detail route or separate AI sidebar. A shared accessible resize
+separator displays a 1px line with a transparent 9px hit area and 8px adjacent
+content padding. It adjusts the list width, initially 320px, with a 240px minimum and at
+least 300px reserved for detail. Pointer dragging and 16px arrow steps/Home/End
+persist `todoPaneWidth` in `goodbuddy.magic-notes-layout.v1`. Narrow layouts hide
+the separator; window constraints do not overwrite the saved desktop width.
+Each source-note heading is a disclosure button with its matching task count.
+Groups start expanded and can independently hide their mounted rows without
+clearing the selected detail. Collapse state lasts while the group is mounted.
+Note detail retains its existing AI pane preferences.
 
 A single secondary button lives in `PageHeader.actions` before New note. Its
 label and icon describe the destination, and `requestDraftSwitch` retains the