feat: add autostart

[franklinkim]

Description

Add a "Start at Login" toggle to the macOS menu bar app, plus README restructuring and a Homebrew cask install hint.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📝 Documentation
• ♻️ Refactoring
• 🏃‍➡️ Performance
• ✅ Tests
• 🔐 Security
• 🔧 Build/CI

Changes

• internal/menubar/tray.go: new "Start at Login" menu entry that reads app.Autostart.IsEnabled(), renders a ☑︎/□ glyph for current state, toggles via Enable()/Disable() on click, and rebuilds the menu; errors are logged as warnings.
• docs/guide/menubar.md: document the new menu entry.
• README.md: move Quick start / Configuration / Use cases below the Documentation + install sections, and document brew install --cask foomo/tap/dockprox-menubar.

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.