Migrate desktop UI from Slint to Tauri + React

[TheSameCat2]

Overview

Replace the Slint-based pikachat-desktop with a Tauri + React + Tailwind frontend while preserving the existing Rust backend crates.

Architecture Principle

Thin Tauri command/event bridge:

• Frontend sends commands: send_message, join_room, load_rooms, etc.
• Backend emits events: timeline_updated, sync_state_changed, login_required, etc.
• Frontend never sees Matrix internals
• Backend never cares about React component structure

This separation enables agents to work independently on frontend and backend.

Tasks

Phase 1: Scaffold Tauri project

• Create apps/pikachat-desktop-tauri with Tauri + Vite + React + TypeScript
• Configure Tailwind CSS
• Set up workspace to include existing crates
• Wire backend-core, backend-matrix, backend-platform as dependencies

Phase 2: Command bridge

• Define Tauri command handlers wrapping existing backend
• init, login_password, restore_session, logout
• start_sync, stop_sync
• list_rooms, open_room, paginate_back
• send_message, send_dm_text
• Media upload/download commands

Phase 3: Event bridge

• Set up Tauri event emission from backend
• timeline_updated, room_list_updated
• sync_state_changed, lifecycle_changed
• login_required, error_occurred

Phase 4: React frontend

• Login screen with homeserver/user/password form
• Room list sidebar
• Timeline view with message bubbles
• Message composer
• Sync status indicator
• Logout flow

Phase 5: Cleanup

• Remove old Slint pikachat-desktop
• Update README and AGENTS.md
• Verify Tailscale serve works for remote testing

Benefits

• Agent can test UI via browser tools (localhost or Tailscale)
• Hot reload for frontend development
• Preserves all existing backend work
• Cleaner separation of concerns