Implement comprehensive high-DPI scaling support for 4K and high-resolution displays

[Copilot]

This PR addresses issue #2855 where users experienced extremely small UI elements (text, buttons, icons) on 4K and high-DPI displays, making Meshroom difficult to use effectively.

Problem

On modern high-resolution displays, Meshroom's UI elements appeared tiny and were hard to read or interact with because:

• No automatic DPI detection or scaling
• Hard-coded font sizes and dimensions throughout the codebase
• No user preferences for UI scaling
• Qt's high-DPI support was not enabled

Solution

Implemented a comprehensive high-DPI scaling system that provides both automatic detection and manual control:

1. Automatic DPI Detection

• Detects logical DPI and device pixel ratio from the primary screen
• Automatically calculates appropriate scaling factors (1x, 1.5x, 2x, 3x, 4x)
• Enables Qt's native high-DPI scaling attributes

2. User Controls

• Display Settings Dialog: Accessible via Edit → Display Settings...
• Independent Scaling: Separate controls for UI elements and fonts (0.5x to 4.0x range)
• Auto-detect Toggle: Switch between automatic and manual scaling modes
• Real-time Preview: Shows scaled UI elements before applying changes
• Reset to Defaults: Restores automatic scaling settings

3. Persistent Settings

• Settings stored using Qt's QSettings system in "Display" category
• Preferences persist between application sessions
• Backward compatible with existing installations

Implementation Details

Core Infrastructure:

• Enhanced meshroom/ui/app.py with DPI detection and scaling management
• Created UISettings.qml singleton providing scaling factors to all QML components
• Added helper functions dp() for dimensions and sp() for fonts

UI Components:

• Updated MaterialToolButton.qml and FloatingPane.qml to use scaled dimensions
• Modified key Application.qml elements to demonstrate scaling
• Created comprehensive DisplaySettingsDialog.qml with preview functionality

User Experience:

• Menu integration in Edit menu
• Display information showing current DPI characteristics
• Intuitive sliders for manual adjustment
• Visual preview of changes before applying

Testing

• ✅ All existing tests continue to pass
• ✅ DPI detection logic verified for common display configurations
• ✅ Settings persistence and loading functionality tested
• ✅ Component updates maintain backward compatibility

Impact

For users on 4K/High-DPI displays:

• Text and UI elements now automatically scale to readable sizes
• Customizable scaling for different preferences
• Consistent experience across all display types

For users on standard displays:

• No impact - defaults to 1.0x scaling
• Existing workflows remain unchanged
• Optional manual controls available if desired

This solution follows Meshroom's existing patterns and conventions, making it easy to extend to additional components as needed. The minimal, surgical changes ensure compatibility while providing significant usability improvements for users on modern high-resolution displays.

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[fabiencastan]

It should have Size in the name, like "smallFontSize" instead of "smallFont".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.46%. Comparing base (bddc006) to head (6c20837).
✅ All tests successful. No failed tests found.

Additional details and impacted files

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  Coverage    77.46%   77.46%           
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  Files           48       48           
  Lines         6735     6735           
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  Hits          5217     5217           
  Misses        1518     1518           

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