Add visual highlighting for selected projects on homepage

[Copilot]

This PR adds visual feedback when a project is clicked on the Meshroom homepage, addressing the lack of UI indication for project selection.

Changes

The implementation adds a red border highlighting system to the project grid view:

• Selection State Management: Added selectedIndex property to track which project is currently selected
• Visual Highlighting: Selected projects now display a 2px red border (#ff0000) with rounded corners
• Interactive Feedback: Selection updates immediately when clicking projects, before opening them
• State Management: Selection is properly cleared when deleting projects or switching tabs

Technical Implementation

The changes are minimal and focused, modifying only meshroom/ui/qml/Homepage.qml:

• Added selectedIndex: -1 property to the GridView component
• Enhanced the Button delegate with conditional border styling using a Rectangle background
• Updated click handlers to set selection state before executing project actions
• Added cleanup logic for invalid selections when the model changes

Visual Effect

Before: No visual feedback when clicking projects

┌─────────┐  ┌─────────┐  ┌─────────┐
│project1 │  │project2 │  │project3 │
└─────────┘  └─────────┘  └─────────┘

After: Clear visual indication of selected project

┌─────────┐  ┏━━━━━━━━━┓  ┌─────────┐
│project1 │  ┃project2┃  │project3 │  ← Red border indicates selection
└─────────┘  ┗━━━━━━━━━┛  └─────────┘

The implementation maintains all existing functionality while providing the visual feedback shown in the issue's reference image.

Fixes #2812.

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