Add dedicated draw settings store and draw_images export folder

[multiplex55]

Motivation

• Rename the draw export directory to draw_images and introduce a dedicated per-executable draw settings file to centralize persistence and enable clearer migration from legacy plugin payloads.
• Keep runtime behavior unchanged by continuing to call runtime().apply_settings(...) whenever settings are loaded or persisted.
• Provide a backward-compatible migration path from the legacy settings.json plugin_settings["draw"] payload when the dedicated file is absent.

Description

• Renamed the export folder constant to DRAW_EXPORT_SUBDIR = "draw_images" while preserving exe-relative resolution and folder auto-creation in src/draw/save.rs and added a test asserting the new folder name.
• Added a new module src/draw/settings_store.rs that resolves draw_settings.json next to the executable and implements load/save helpers plus load_from_path/save_to_path with parent-folder creation, contextual errors, and sanitization; the load path will migrate from legacy settings.json plugin payload when the dedicated file is missing.
• Registered the new module in src/draw/mod.rs and updated src/gui/draw_settings_dialog.rs to use settings_store::load(...) and settings_store::save(...) for dialog load/save flows while still writing through to Settings::plugin_settings for transitional compatibility and calling runtime().apply_settings(...).
• Updated src/plugins/draw.rs so plugin initialization prefers the dedicated store, apply_settings checks the dedicated store first (then falls back to the provided plugin payload), and persist_settings writes the dedicated store as well as producing the plugin UI payload; also adjusted settings_ui to use the plugin's in-memory settings.
• Added/updated unit tests: src/draw/save.rs (folder name assertion), src/draw/settings_store.rs (path resolution, round-trip serialization, legacy migration), GUI tests in src/gui/mod.rs (dedicated-file persistence and migration), and plugin tests in src/plugins/draw.rs (dedicated-store preference and legacy migration).

Testing

• Ran cargo fmt successfully to format changes.
• Attempted to run unit tests, but full test execution failed in this environment due to a system dependency needed by the build (alsa-sys / missing alsa.pc for pkg-config), which caused cargo test to abort during crate compilation, so added tests were not fully executed here.
• The new tests added are: draw::save::tests::export_folder_name_matches_draw_images_contract, draw::settings_store::tests::settings_path_is_resolved_next_to_executable, draw::settings_store::tests::dedicated_store_roundtrip_serialization, draw::settings_store::tests::load_migrates_legacy_plugin_settings_when_dedicated_file_is_missing, GUI tests in gui::tests for dedicated-file persistence and migration, and plugin tests plugins::draw::tests::apply_settings_prefers_dedicated_store_when_present and plugins::draw::tests::apply_settings_migrates_from_legacy_plugin_settings_when_dedicated_missing, which should pass when run in an environment with required system libraries installed.

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Codex Task