Add theme preference with Light/Dark/System selection

[Copilot]

Adds user preference for light/dark mode theme selection under Settings → Preferences → Theme.

Changes

Config persistence

• New theme field in config file with values: "light", "dark", "system" (default)
• Validation on load/save with fallback to system for invalid values

Theme application

• apply_theme() function sets Qt QPalette colors for light/dark modes
• System option resets to platform defaults
• Applied immediately on selection (no restart required)
• Applied on startup from saved preference

Menu structure

• New "Preferences" submenu under Settings menu
• "Theme" submenu with mutually exclusive checkable options via QActionGroup

Screenshots

Dark Theme

Light Theme

Tests

14 new tests in test_theme.py covering persistence, validation, backward compatibility, and color specifications. All 67 tests pass.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Add option to change theme</issue_title>
<issue_description>## Goal

Add user preference for light/dark mode theme selection, with options to explicitly choose light, dark, or follow system default.

Success Criteria

• New "Preferences" submenu exists under the Settings menu
• Preferences submenu contains theme selection with three options: Light, Dark, System (default)
• Selecting Light or Dark applies that theme immediately, regardless of system setting
• Selecting System respects the OS light/dark mode preference (current behavior)
• Theme preference persists across application restarts via existing config file
• Application loads with correct theme on startup based on saved preference
• All existing tests pass
• New tests cover theme preference persistence and application

Constraints

• Use existing config file mechanism for persistence—do not introduce new storage
• Do not modify existing Settings menu items; add new Preferences submenu alongside them
• Out of scope: custom themes, accent colors, scheduled theme switching

Context

• Existing settings/config persistence: (wherever current settings are saved)
• Settings menu implementation: (wherever the current menu is defined)

Notes

• "System" should be the default for new installations to preserve current behavior
• Consider whether theme change requires restart or can apply live (live preferred if feasible)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add option to change theme #70

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