feat: persist per-preset stage collapse state

[OpenSauce]

Summary

• Persist each preset's stage collapsed/expanded layout in settings.json as a HashMap<preset_name, Vec<bool>>
• Collapse state is restored when switching presets or restarting the app
• State is saved on toggle, add, remove, and reorder operations
• Stale entries are cleaned up on preset deletion
• Skips persistence when no preset is selected to avoid orphaned entries
• Backwards compatible — existing settings files without the field deserialize with an empty map

Test plan

• make lint passes
• make test passes (62 tests)
• Collapse some stages, restart app — verify they remain collapsed
• Switch between presets — verify each preset remembers its own collapse state
• Add/remove/reorder stages — verify collapse state updates correctly
• Delete a preset — verify its collapse entry is removed from settings.json

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds persistence of the stage collapsed/expanded UI state on a per-preset basis by storing it in settings.json, so the layout is restored when switching presets or restarting the app.

Changes:

• Add collapsed_stages: HashMap<String, Vec<bool>> to Settings with a default empty map.
• Restore collapsed state per preset during app boot and when loading stages for a preset.
• Persist collapsed state on toggle/add/remove/reorder and clean up state on preset deletion.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/settings/mod.rs	Extends Settings to serialize/deserialize per-preset collapsed stage state.
src/gui/app.rs	Restores/persists collapse state across preset switches and app restarts; removes stale state on preset deletion.

Comments suppressed due to low confidence (1)

src/gui/app.rs:431

• On PresetMessage::Delete, the settings are updated to selected_preset = None when the deleted preset was selected, but PresetHandler::delete_preset will auto-select the first remaining preset. This leaves settings and UI out of sync; it also means collapse state restoration/persistence will use the empty key and effectively stop persisting until the user manually re-selects a preset. Update settings.selected_preset to the handler's newly selected preset after deletion (or change the flow so settings is always kept in sync with PresetHandler).

                    PresetMessage::Delete(deleted_name) => {
                        self.settings.collapsed_stages.remove(&deleted_name);
                        if self.settings.selected_preset == Some(deleted_name) {
                            self.settings.selected_preset = None;
                        }
                        self.save_settings();

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