Improve configurator workflows, palette navigation, and rendering performance

[devmobasa]

Protect unsaved configurator edits during save, reload, and close operations, improve keyboard interaction, and reduce repeated work in shortcut analysis, session snapshots, and page thumbnails.

• Block conflicting configurator actions during document operations and preserve drafts when leaving.
• Keep command palette selection visible in small viewports, apply the resolved theme, and skip idle repeat preparation.
• Add keyboard and accessibility support to GTK sliders, including Escape focus release.
• Reuse parsed shortcut data, copy only retained session history, and cache thumbnails with content invalidation. Solid pages with magnified Spotlights use direct rendering to preserve sampling beyond page edges.
• Consolidate palette, capture, and save state ownership; share slider and board-validation policy.
• Align CI with local validation and require isolated GTK widget coverage.

Validation: full local checks passed in both feature configurations for the implementation batch, along with 70 isolated GTK widget tests. The final Spotlight fix additionally passed the thumbnail suite and strict workspace Clippy in both feature configurations, plus formatting and whitespace checks. Its pixel regression test covers all four edges, repeated paints, partial clips, solid and transparent boards, ARGB32/RGB24 targets, and device/matrix scaling.