doc: Add dark theme implementation plan

[Copilot]

Documents the plan for adding app-controlled dark theme support to oedit/otbedit (MFC-based) without requiring OS-level dark mode.

Added: doc/dark_theme_plan.md

• Option flag — BOOL dark_theme in _st_option with INI persistence
• Title bar — DwmSetWindowAttribute(DWMWA_USE_IMMERSIVE_DARK_MODE) per-app, Windows 10 Build 18985+
• Dialogs/controls — WM_CTLCOLOR overrides via shared CDarkModeUtil utility in octrllib
• Toolbar/statusbar — SetWindowTheme(L"DarkMode_Explorer", NULL)
• Editor colors — leverage existing solarized-dark preset in PresetColor.h; auto-apply on theme switch

Scope/Limitations

Target	Feasibility
Title bar, dialogs, edit controls, listboxes	✅ Full
Toolbar / statusbar	✅ Full
Push buttons	⚠️ Owner-draw required
Menus	⚠️ Complex, needs further investigation
Common dialogs (file open, etc.)	❌ OS-controlled