Unify toolbar icons across built-in and GTK frontends

[devmobasa]

Summary

• Replace the mixed toolbar icon implementations with a cohesive 24×24 procedural Cairo icon family.
• Share the same icon entry points across the built-in and GTK toolbar frontends.
• Refresh tool, shape, action, history, drag, pin, overflow, minimize, and restore icons.
• Give the shape picker a dedicated family icon instead of displaying the last-used shape.
• Use directionally appropriate collapse icons for the horizontal top toolbar and vertical side toolbar.

Rendering

• Preserve the caller’s foreground color and opacity for hover, active, disabled, and destructive states.
• Keep icons vector-scaled without introducing runtime SVG assets or SVG-rendering dependencies.
• Render the built-in drag glyph at the same intended size as the GTK version.

Validation

• Added rendering coverage for all icon entry points at 18, 20, 22, 24, and 28 pixels.
• Verified minimized top and side toolbar behavior.
• Passed the complete ./tools/lint-and-test.sh suite with all features and without default features.