Add configurable query-results grid layout with keyboard navigation

[multiplex55]

Motivation

• Provide an optional grid layout for query results so users can view results in rows/columns instead of a single vertical list.
• Make layout selection plugin-aware so plugins can advertise compatibility and the UI can fall back to a list when appropriate.
• Allow users to configure the grid (enable/rows/cols) from the settings UI and have changes apply immediately at runtime.
• Improve keyboard navigation to support 2D movement in grid mode (arrow keys + numpad mapping).

Description

• Settings: added QueryResultsLayoutSettings to src/settings.rs with serde defaults and a Default impl, and wired it into Settings::default() and (de)serialization so existing configs remain compatible. (fields: enabled, rows, cols, respect_plugin_capability, plugin_opt_out).
• Settings editor: extended SettingsEditor in src/settings_editor.rs with fields for the layout, populated them in SettingsEditor::new(...), converted back in to_settings(...) (rows/cols clamped to >=1), and added UI controls (toggle, numeric rows/cols, capability-respect toggle and comma-separated plugin opt-out). Changes are applied to runtime LauncherApp immediately when saved.
• Runtime and rendering: added query_results_layout + resolved_grid_layout to LauncherApp in src/gui/mod.rs, recompute hooks (should_use_grid_layout / recompute_query_results_layout), and wire recompute calls into search and settings-apply flows so edits take effect immediately. Implemented a grid rendering path (egui::Grid) for the main query results pane while preserving the existing vertical list as a fallback and handling overflow/scrolling naturally.
• Plugin-aware gating: added capability markers in src/plugin.rs (grid_results_compatible and force_list_results) and implemented logic in LauncherApp::should_use_grid_layout that resolves grid vs list based on active plugins, capability presence, and the opt-out list; mixed or ambiguous sources fall back to list mode.
• Keyboard navigation: extended LauncherApp::handle_key(...) to support Left/Right and numpad 2/4/6/8 for 2D navigation in grid mode, while preserving list behavior in list mode and keeping Enter activation semantics.
• Tests: added focused unit tests covering settings serde/defaults and roundtrip, settings-editor conversion roundtrip & clamping, grid keyboard navigation (arrows + numpad), and plugin-aware layout selection logic (all added in src/settings.rs, src/settings_editor.rs, and src/gui/mod.rs test modules).

Testing

• Ran cargo fmt to format the code; it completed successfully.
• Attempted to run unit tests (compiled a targeted test group), but the build aborted due to a missing system library required by a dependency (alsa): running cargo test in this environment failed with a pkg-config/alsa.pc error from alsa-sys during compilation, so automated tests could not be completed here.
• Unit tests added: settings defaults & round-trip (src/settings.rs), settings-editor conversion & clamping (src/settings_editor.rs), grid navigation and plugin capability/resolution (src/gui/mod.rs), intended to pass when dependencies and native build tools are available.

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Codex Task