Dev Workflow: Local Supabase Stack & Impersonation

Overhaul of local development workflow. Modes 2 & 3 now auto-start Docker + supabase local, replacing the previous native-Postgres-only approach.

• 🐳 Auto Docker + Supabase: Sync and local modes now start Docker Desktop and `supabase start` automatically. The local Supabase REST API (port 54331) is required because the app uses `@supabase/supabase-js` SDK.
• 🔀 Impersonate Users: Replace the email/password login form with a searchable user dropdown with autocomplete. Select any user from the local DB to log in instantly — no password needed. Only available on localhost.
• 📦 Documentation Consolidation: 10 root doc files merged into 1 README + docs/ subdirectory. Cleanup commit removed stale migration docs and tiny bug lists.