feat: v0.4 — implement PyInstaller desktop EXE build and GitHub Actions release pipeline

[Codex-Crusader]

Summary

The ROADMAP.md v0.4 milestone targets a desktop EXE so users can "download and double-click" without any terminal interaction. This issue tracks the full v0.4 deliverable set.

Deliverables (from ROADMAP.md v0.4)

• PyInstaller EXE — Windows-first, then macOS and Linux
• Launcher script (pulseengine/local/launcher.py) — starts the Streamlit server, waits for it to be ready, then opens the browser automatically
• System tray icon — shows running status, allows clean shutdown via tray menu
• GitHub Actions pipeline — builds and attaches platform binaries to every release tag
• First-run setup wizard (pulseengine/local/setup.py) — lightweight UI to configure the data directory on first launch

Technical notes

• The launcher script is already referenced in the ROADMAP target repo structure but the file does not yet exist in pulseengine/local/
• The GitHub Actions workflow should produce artefacts for windows-latest, macos-latest, and ubuntu-latest runners
• Consider using pystray for the system-tray integration and pyinstaller --onefile --windowed for the EXE
• The builds/ directory (already git-ignored) should be the target output folder

Acceptance criteria

1. Running the Windows EXE opens the dashboard in the default browser with no terminal required
2. The tray icon shows a "Stop" option that shuts down the Streamlit server cleanly
3. A GitHub Actions release workflow attaches the EXE to tagged releases automatically