fix: keep Treeland screenshot toolbar on screen

[svan71]

Summary

• choose the monitor that owns the Treeland capture region
• place the screenshot toolbar below or above the selection when space permits
• clamp new and restored toolbar positions to the selected monitor bounds
• add focused geometry coverage for screen edges and multi-monitor layouts

Why

The Treeland screenshot toolbar used a position derived from the capture region and the main overlay size without checking the actual monitor bounds. Near the top or right edge, controls could overlap the selected image or extend off-screen.

Testing

• focused geometry test executable: 10/10 tests passed
• git diff --check passed
• qmake full-tree compile reached an existing current-master error in main_window.cpp: QPainterPath is incomplete because its header is not included; the new placement helper and its direct consumers compiled before that unrelated failure

This change contains no distribution theme or user-specific customization.

[deepin-ci-robot]

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This pull-request has been approved by: svan71

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[deepin-ci-robot]

Hi @svan71. Thanks for your PR. 😃

[deepin-ci-robot]

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