refactor: restructure into core/desktop/cli workspace

[sujithatzackriya]

Summary

• Extracts shared logic into core/ crate (notification manager, types, HTTP server, waiters, webhook, notifier trait)
• Moves Tauri desktop app from root src-tauri/ into desktop/ workspace member, frontend from src/ to desktop/frontend/
• Adds CLI auto-launch daemon — tries macOS app bundle first, falls back to headless server
• Workspace: core, cli, desktop as members with shared syncfu-core dependency

Structure

core/          — syncfu-core shared library
cli/           — syncfu CLI (talks to server via HTTP)
desktop/       — Tauri desktop app (tray, overlay, HTTP server)

Test plan

• cargo check passes for all 3 workspace members
• cargo test -p syncfu-core — core unit tests
• cd desktop && pnpm tauri build — desktop app builds
• cargo build -p syncfu-cli — CLI builds
• Desktop app starts (tray icon, server on :9868)
• CLI auto-launches desktop app when server is down

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