Refine layout config editing and add shared config-file helper

[multiplex55]

Motivation

• Ensure plugin "Edit config" actions resolve file paths relative to the configured settings location instead of using hard-coded defaults.
• Create missing config files from a default template and surface clear feedback about the exact path opened/created.
• Centralise path resolution/creation logic to avoid per-plugin duplication.

Description

• Add a shared helper src/common/config_files.rs providing ConfigFileSpec, resolve_config_path and ensure_config_file to resolve config paths relative to the settings file and create missing files with default templates, and include unit tests for the helper.
• Introduce a settings path registry with set_settings_path/settings_path in src/settings.rs and call set_settings_path("settings.json") at startup in src/main.rs so helpers can resolve relative config locations.
• Wire the layouts subsystem to the shared helper: add DEFAULT_LAYOUTS_TEMPLATE and LAYOUTS_CONFIG in src/plugins/layouts_storage.rs, expose layouts_config_path() and update load_layouts/save_layouts to accept Path-style arguments and ensure parent directories are created on save.
• Update layout actions and UI to use the resolved settings-based layout path and to present a clear Create/Edit label with the resolved path; changes in src/actions/layout.rs, src/plugins/layout.rs, and src/dashboard/widgets/layouts.rs use ensure_config_file to create templates when missing and open::that the exact file path.

Testing

• Added unit tests exercising path resolution and create/open behavior in src/common/config_files.rs and attempted to run them with cargo test config_files.
• The test run compiled dependencies but the build failed in this environment due to a missing system library required by alsa-sys (error: missing alsa.pc / alsa system library), so the unit tests were not executed here; the helper tests are self-contained and should run when the environment can build all crates or when running the specific package tests without the system-dependent crates.

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Codex Task