- Single source of truth for layout sizes in constants.ts
- Store type: twoPanel changed from number[] to number (px)
- tagsListHeight migration from percentage to pixels

Supports horizontal and vertical drag with cursor, selection
blocking, and data-resizing attribute for active state styling.

Unified 2-panel and 3-panel modes via flex layout. Removed reka-ui
Splitter dependency. All sizes from LAYOUT_DEFAULTS.

Removed percentage-based twoPanel logic. Both now use pixel-based
store values with LAYOUT_DEFAULTS fallbacks via ThreeColumn.

Tags panel now uses pixel-based height with useResizeHandle composable
instead of reka-ui SplitterGroup percentages.

…ebarFolders

Same pattern as Library.vue — tags panel uses pixel-based height
with useResizeHandle composable.