feat: add sidebar project drag-and-drop reordering

- Add sidebarPosition column to projects table with migration
- Add PUT /projects/reorder endpoint for persisting order
- Update sidebar query to sort by sidebarPosition
- Add drag handles and DnD logic to AppSidebar ProjectGroup
- Add reorderProjects API client method
- Include light mode design spec

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

Author: Remote Agent

docs/superpowers/plans/2026-03-28-light-mode.md

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+ALTER TABLE "projects" ADD COLUMN "sidebar_position" integer DEFAULT 0 NOT NULL;