Harden settings dialog activation and error handling

[multiplex55]

Motivation

• Prevent recursive/query-chaining when opening settings which could retrigger the command search and cause crashes.
• Make the settings dialog open flow resilient to malformed or unreadable settings files so the app doesn't panic.
• Surface read/save failures to users via the existing toast/error system instead of silently failing or panicking.

Description

• Change the settings command action from query:settings to a direct settings:dialog in src/plugins/settings.rs.
• Introduce LauncherApp::open_settings_dialog() in src/gui/mod.rs to load Settings safely and initialize SettingsEditor only when the dialog is not already shown, and to surface load errors via set_error/toasts.
• Replace direct self.show_settings = true sites with calls to open_settings_dialog() so all openings use the guarded flow.
• Surface settings read failures during save in src/settings_editor.rs by adding toast/error feedback instead of a plain set_error.

Testing

• No automated tests were executed as part of this change.
• Changes were validated by static inspection of the modified call sites and ensuring no remaining query:settings usages remain.
• The new guarded path is implemented to avoid reloading the editor when show_settings is already true.
• Error-toasting follows the existing Toast APIs so failures will be visible to users at runtime.

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Codex Task