Migrate backend to Encore and local desktop auth

From the PR description

User description

Summary by Sourcery

Migrate authentication and backend infrastructure from Supabase to an Encore-managed Postgres and local JWT-based auth system, and add an optional Tauri-based desktop shell alongside build and tooling updates.

New Features:

• Introduce a local JWT-based authentication API with signup, login, session, refresh, and logout endpoints backed by a users table.
• Add a Supabase-to-Encore migration script and initial Encore SQL migration defining core application tables and legacy user mapping.
• Add a Tauri v2 desktop shell that runs a packaged Next.js standalone server via a sidecar binary for desktop builds.

Bug Fixes:

• Ensure account deletion also removes or unlinks associated application data and legacy user claims before deleting the user record.
• Normalize and prefix R2 storage keys to avoid collisions and support configurable key prefixes across uploads, downloads, deletes, and signed URLs.

Enhancements:

• Replace Supabase client usage in the frontend with a lightweight local auth client that talks to the new backend auth endpoints and stores sessions in localStorage.
• Replace the server-side Supabase client with a Postgres-backed compatibility layer and Encore SQLDatabase, keeping existing route shapes while migrating off Supabase.
• Refactor auth middleware and user extraction to validate Mike bearer tokens instead of calling Supabase.
• Adapt the Express app to run under Encore via a raw API shim while preserving standalone server startup for local development.
• Add Postgres client abstractions and tests to emulate the Supabase query builder API against raw SQL.
• Improve safe local testing docs and README to describe the Encore setup, new env vars, migration flow, and desktop build commands.

Build:

• Add Encore, pg, and related typings to the backend, introduce a test script and a Supabase-to-Encore migration script entrypoint.
• Add Tauri CLI and pkg-based sidecar build tooling to the frontend, plus a desktop build flag that produces a Next.js standalone output and wiring for sidecar preparation scripts.

Documentation:

• Update README and safe-local-testing docs to reflect Encore-based backend, local auth, database migration steps, desktop shell usage, and revised environment variables.

Tests:

• Add unit tests for the Postgres Supabase-compat client and its SQL generation behavior.

CodeAnt-AI Description

Move Mike to local auth, Encore PostgreSQL, and desktop support

What Changed

• Sign-up, login, session refresh, and logout now use Mike's own auth flow instead of Supabase, with sessions stored locally in the app.
• The app now loads user data from Encore PostgreSQL and keeps the same main API routes while running through an Encore-backed backend.
• Deleting an account now clears associated project, document, workflow, chat, and profile data before removing the user.
• Storage keys can now be kept under a configurable R2 prefix, and migrated files avoid double-prefixing.
• A desktop version is now available through Tauri, with build and launch support for packaged Windows, macOS, and Linux apps.
• The migration tool can copy legacy Supabase data and R2 objects into the new Encore setup and write a report of what was moved or skipped.

Impact

✅ Sign in without Supabase ✅ Safer account deletion ✅ Cleaner data migration ✅ Desktop app builds

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