feat(ui): restructure SMF menu layout with section headers and spacing

[codepuncher]

Summary

• Adds SeparatorText headers (General, Start button, Back button) to divide the flat settings list into named sections
• Adds Spacing() between sections and after each header for breathing room
• Wraps MCM sub-options (mod name combo + close journal checkbox) in Indent()/Unindent() to visually attach them to the action combo above
• Simplifies action combo labels from "Start long-press action" / "Back long-press action" to "Action" (section header carries the context)
• Adds Separator() + Spacing() above the button row
• All changes confined to RenderSettings() in src/MenuUI.cpp; no logic changes

Test plan

• Tested in-game — layout renders correctly with section headers, spacing, and indented MCM sub-options
• Save, reload, and reset all function correctly

[Copilot]

Pull request overview

This PR refactors the in-game settings menu layout in RenderSettings() to improve readability by grouping related controls into labeled sections, adding spacing/separators, and visually nesting MCM sub-options under their parent action.

Changes:

• Added section headers (General, Start button, Back button) using ImGuiMCP::SeparatorText() and added Spacing() to improve scanability.
• Indented MCM-related sub-options with Indent()/Unindent() so they visually attach to the corresponding action selection.
• Simplified action combo visible labels to Action (with unique ImGui IDs via ##start / ##back) and added a separator + spacing above the Save/Reload/Reset button row.

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