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/db/src/schema/projects.ts

@@ -14,6 +14,8 @@ export const projects = pgTable(
     status: text("status").notNull().default("backlog"),
     leadAgentId: uuid("lead_agent_id").references(() => agents.id),
     targetDate: date("target_date"),
+    color: text("color"),
+    archivedAt: timestamp("archived_at", { withTimezone: true }),
     createdAt: timestamp("created_at", { withTimezone: true }).notNull().defaultNow(),
     updatedAt: timestamp("updated_at", { withTimezone: true }).notNull().defaultNow(),
   },