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/migrations/0015_project_color_archived.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "projects" ADD COLUMN "color" text;
+ALTER TABLE "projects" ADD COLUMN "archived_at" timestamp with time zone;

packages/db/src/migrations/meta/_journal.json

@@ -99,6 +99,20 @@
       "when": 1771623691139,
       "tag": "0013_dashing_wasp",
       "breakpoints": true
+    },
+    {
+      "idx": 14,
+      "version": "7",
+      "when": 1771691806349,
+      "tag": "0014_many_mikhail_rasputin",
+      "breakpoints": true
+    },
+    {
+      "idx": 15,
+      "version": "7",
+      "when": 1771865100000,
+      "tag": "0015_project_color_archived",
+      "breakpoints": true
     }
   ]
 }