Layer display, settings window separation, and i18n support

[shinya]

Summary

• Layer overlay display (transparent overlay + draggable control bar)
• Settings window separated into its own entry point
• i18n support (English / Japanese, auto-detect)
• Improved UI icons (SVG) for display mode, layer toggle, and fullscreen
• Per-window capability-based permission separation

Changes

• Layer display: Added layer.html (transparent click-through overlay) and layer_ctrl.html (drag handle). NSPanel class swap for all-Spaces visibility on macOS
• Settings window: Extracted settings into a separate Vite entry point (settings.html / SettingsApp.tsx). Syncs with main window via Tauri events
• i18n: Added src/i18n/ with en/ja translations. Language setting supports auto/en/ja
• UI icons: Replaced Unicode characters with inline SVGs for display mode, layer, and fullscreen toggle buttons
• Rust backend: Added overlay behavior, layer position sync, and window management commands