feat: add inline project timeline and notes section to Project Detail

- Implemented a Gantt timeline using GanttChart component scoped to project checkpoints.
- Added functionality to create and manage checkpoints with AddCheckpointDialog.
- Introduced EditCheckpointDialog for editing existing checkpoints.
- Created a notes section displaying a list of notes with the ability to add new notes.
- Updated routing to include notes detail page.
- Enhanced GanttChart with context menu for editing and deleting checkpoints.
- Improved UI components for better user experience.

progress.txt

@@ -249,3 +249,23 @@
   - When grouping tasks by status for Kanban columns, always handle unknown/null status values with a fallback to prevent tasks from disappearing
   - `DragDropContext.onDragEnd` provides `draggableId` which maps directly to `task.id` — no need to look up the task object for status updates
 ---
+
+## 2026-02-22 - US-015
+- What was implemented:
+  - Added Project Timeline section to `ProjectDetail.tsx` between AI Summary and Tasks Kanban
+  - Reused `GanttChart` component (from US-012) scoped to current project's checkpoints
+  - "+ Add" Button (variant=outline, size=sm) opens `AddCheckpointDialog` with `defaultProjectId={projectId}` (hides project selector)
+  - Wired `checkpoints.delete` mutation with `onDelete` prop for checkpoint dot deletion
+  - Computed `ganttStart`/`ganttEnd` dynamically from checkpoint dates with 1-month padding (fallback ±2 months if empty)
+  - Added Notes section below Tasks Kanban using `Item` component (variant=muted) in a flex-wrap grid layout matching Figma design
+  - Each note card shows `SquareDashed` icon + title + formatted createdAt date, clickable to navigate to `/notes/$noteId`
+  - "+ Add" Button calls `notes.create({ title: 'Untitled Note', content: '', projectId })` then navigates to the new note
+  - Created route stub at `src/renderer/routes/notes.$noteId.tsx` with back button + note title placeholder (full editor deferred to US-016)
+- Files changed: `src/renderer/components/projects/ProjectDetail.tsx`, `src/renderer/routes/notes.$noteId.tsx` (new), `prd.json`, `progress.txt`
+- **Learnings for future iterations:**
+  - GanttChart + AddCheckpointDialog are designed for reuse: `defaultProjectId` prop scopes the dialog to a project and hides the project select dropdown
+  - Figma notes section uses a card grid layout (flex-wrap with Item cards), not a flat list with Separators — always cross-reference Figma when acceptance criteria text diverges
+  - `trpc.useUtils()` provides `invalidate()` for cache busting after mutations — use at the component level, not inside mutation callbacks
+  - `notes.create` returns `{ id }` which can be used directly for navigation in the `onSuccess` callback
+  - TanStack Router file-based routing: `notes.$noteId.tsx` generates `/notes/:noteId` route automatically — `Route.useParams()` provides typed `{ noteId }`
+---