Add action metadata and selected-action preview panel

[multiplex55]

Motivation

• Provide richer context for launcher results so users can see a human-friendly preview, risk level, and optional icon for actions.
• Surface metadata for high-impact operations (system, recycle, layout, task manager) to reduce accidental destructive actions.
• Let users toggle the preview UI and choose a compact presentation from settings so the feature is optional and non-intrusive.

Description

• Extend the action model by adding preview_text: Option<String>, risk_level: Option<ActionRiskLevel>, and icon: Option<String> plus a new ActionRiskLevel enum for Low/Medium/High/Critical in src/actions/mod.rs.
• Populate metadata for high-impact plugins and their command shortcuts in src/plugins/system.rs, src/plugins/task_manager.rs, src/plugins/recycle.rs, and src/plugins/layout.rs so returned Actions carry preview text, a risk classification, and an icon identifier.
• Update the main launcher UI (src/gui/mod.rs) to render a split view showing the results list and a preview pane when an item is selected and preview_enabled is true, with compact/expanded behavior controlled by preview_compact_mode.
• Add settings fields preview_enabled and preview_compact_mode to Settings and wire them into the settings editor so the preview panel can be enabled/disabled and persisted; propagate those settings into LauncherApp state.
• Add/adjust many action constructors and widget code paths to initialize new Action fields (defaulting to None where appropriate) and add new unit tests that assert metadata presence and selection/visibility behavior.

Testing

• Ran cargo fmt successfully to format the changes.
• Attempted cargo check and running the test suite, but cargo check failed in this environment due to a Rust toolchain mismatch (installed rustc 1.87.0 while a dependency requires 1.88.0), so automated compilation/tests could not complete here.
• Added/updated unit tests covering selection behavior and plugin metadata expectations (examples: tests/selection.rs, tests/gui_visibility.rs, tests/system_plugin.rs, tests/task_manager_plugin.rs, tests/recycle_plugin.rs, tests/layout_plugin.rs), but their execution was blocked by the toolchain constraint.
• Committed all changes after formatting; the change set includes model, plugin, UI, settings, and test updates described above.

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Codex Task