UI: compact layer controls, layers/details popouts, and preview for selected layers

[Alyndiar]

Motivation

• Make the left-side workspace more compact and usable by simplifying layer/path action buttons, ensuring visible scrollbars and providing popout dialogs for the Layers and Details sections.
• Show a simple preview when a layer is selected (use a default hole position/angle) so users can inspect a layer without selecting a path.
• Rebalance left panel layout so Layers/Details/Preview occupy roughly 40%/40%/20% of the available height and center the preview display.

Description

• Replace the horizontal action row in LayerListPanel with a two-row QGridLayout and compact symbolic labels, set QSizePolicy.Minimum for these buttons, and add tooltips mapped to the original texts via _update_button_labels (buttons: +L, +P, ✎, ✓/✕, ↑, ↓, T, -, ✓A/✕A).
• Force visible vertical scrollbars for the layers tree and main left QScrollArea by setting setVerticalScrollBarPolicy(Qt.ScrollBarAlwaysOn) and hide horizontal scrollbars where appropriate.
• Add header bars with a popout QToolButton (⤢) to layers_group and details_group and implement _open_section_popout / _restore_section_popout to move a group into a QDialog and restore it on close.
• Convert the details area to a QScrollArea with always-on vertical scrollbar and keep preview centered in its area; set left panel widget stretches when adding to the left layout (layers_group stretch=4, details_group stretch=4, preview_group stretch=2) to approximate 40%/40%/20% vertical allocation.
• Make SpiroWindow._refresh_preview render when a layer is selected (not only path) by treating the hole offset/direction as 0.0 and applying path_zoom only when a path is selected, leaving other math unchanged and reusing existing pen/wheel computations.

Testing

• No automated tests were executed for this change (UI-only modifications).

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Codex Task