Add comprehensive Settings dialog

[an0nn30]

Summary

• Adds an IntelliJ-style Settings dialog (Cmd+,) with grouped sidebar navigation and 7 content sections: Appearance, Keyboard Shortcuts, Terminal, Shell, Cursor, Plugins, and Advanced
• Exposes all config.toml options in a GUI — settings are read/written to the same config.toml file
• Absorbs the standalone Plugin Manager dialog into the Plugins section
• Settings that can be hot-reloaded (theme, appearance mode, UI fonts, keyboard shortcuts) apply immediately on Apply; others show a "restart required" toast
• Includes a keyboard shortcut recorder for rebinding shortcuts
• Backend: new settings.rs module with get_all_settings, save_settings, list_themes commands and needs_restart logic (7 unit tests)
• TauriState.config wrapped in Mutex for runtime mutability
• Legacy [font] section suppressed from serialization output

Test plan

• Cmd+, opens Settings dialog; Cmd+, again toggles it closed; Escape closes it
• Each sidebar section displays correct current values from config.toml
• Change theme + Apply — theme changes immediately
• Change terminal font + Apply — toast: "Some changes require a restart to take effect"
• Cancel after making changes — reopen shows original values
• Plugins section: enable/disable a plugin, rescan, verify functionality matches old Plugin Manager
• Keyboard shortcuts: click a shortcut, record a new binding, Apply, verify menu accelerator updates
• New window (Cmd+Shift+N) — Settings menu item works there too
• View menu still has Zoom In/Out/Reset and all other items after plugin load (no menu drift)