Folder-based launch volume rules

• Set a default for a folder - Pick a folder and new desktop apps launched from it start at the volume you choose. This is the folder-based follow-up to issue #30.
• Nested folders do what you expect - If rules overlap, the deepest matching folder wins, so a more specific game or tools folder can override a broad Steam, Work, or Projects rule.
• Explicit app rules still win - A per-app Set at launch or Lock rule stays more specific than any folder default. Hard mute also keeps composing normally with the result.
• The rules are editable and portable - Add, browse, edit, and remove folder rules from Settings. They are stored with the rest of the profile and included in settings export/import.

Small feature, but a pretty useful one if your games, tools, or work apps tend to start way louder than you would like. Thanks to everyone who kept pushing on this request.