feat: foldable PROJECTS section in sidebar with color support

- Add `color` (text) and `archivedAt` (timestamp) columns to projects table
- Add PROJECT_COLORS palette constant (10 colors) in shared package
- Add color/archivedAt to Project type interface and Zod validators
- Auto-assign next available color from palette on project creation
- New SidebarProjects component with:
  - Collapsible PROJECTS header above WORK section
  - Caret toggle visible on hover (left of header)
  - Always-visible plus button (right of header) opens NewProjectDialog
  - Lists non-archived projects with colored rounded squares
  - Active project highlighted based on URL match
- Remove Projects nav item from WORK section in sidebar

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/shared/src/validators/project.ts

@@ -10,6 +10,8 @@ export const createProjectSchema = z.object({
   status: z.enum(PROJECT_STATUSES).optional().default("backlog"),
   leadAgentId: z.string().uuid().optional().nullable(),
   targetDate: z.string().optional().nullable(),
+  color: z.string().optional().nullable(),
+  archivedAt: z.string().datetime().optional().nullable(),
 });
 
 export type CreateProject = z.infer<typeof createProjectSchema>;