Refactor theme settings dialog into a two-column editor

[multiplex55]

Motivation

• Reduce excessive vertical stacking in the Theme settings dialog and make related controls readable side-by-side.
• Improve color-row compactness for easier comparison and editing of RGBA values.
• Keep existing section-level "Reset section defaults" semantics and the action bar behavior unchanged while making reset logic clearer.
• Add unit tests for non-UI logic to guard against regressions in defaults and section resets.

Description

• Widened the Theme settings window by changing the default width to 980.0 and reorganized the content into a two-column layout using ui.columns(2, ...) in src/gui/theme_settings_dialog.rs.
• Placed section_base_mode, section_core_surfaces, and section_text_and_links in the left column, and section_widgets, section_selection, section_semantic plus preview in the right column.
• Extracted section-reset logic into focused helpers (reset_core_surfaces_to_mode_defaults, reset_text_and_links_to_mode_defaults, reset_widgets_to_mode_defaults, reset_selection_to_mode_defaults, reset_semantic_to_mode_defaults) and wired existing "Reset section defaults" buttons to these helpers without changing their semantics.
• Made color_row(...) more compact by adding a fixed LABEL_WIDTH, setting a minimum row height, and tightening horizontal spacing while preserving color_edit_button_srgba RGBA editing behavior.
• Added a #[cfg(test)] mod tests with unit tests that cover defaults_for_mode(...) and regression tests asserting each section-reset helper updates only its intended fields.

Testing

• Ran cargo fmt --all which completed successfully.
• Executed cargo test theme_settings_dialog -- --nocapture but the build failed due to a missing system dependency required by a transitive crate (alsa-sys failed looking up alsa.pc via pkg-config), so the newly added unit tests could not be run in this environment.
• The added tests are pure logic/state-transition checks and should pass in a development environment where system dependencies for the full crate build are available.

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Codex Task