feat(FN-3089): add draggable dependency graph nodes

Adds draggable interaction to dependency graph nodes (FN-3089), introducing a `useNodeDrag` hook to manage drag state and positioning, with corresponding tests and a new drag stylesheet; the `GraphTaskNode` and `DependencyGraph` components are updated to wire up the drag behavior.

Fusion-Task-Id: FN-3089

plugins/fusion-plugin-dependency-graph/README.md

@@ -9,7 +9,7 @@ Plugin-provided top-level **Graph** dashboard view for Fusion.
 - **Orphan dependency handling**: if a visible task depends on an excluded/missing dependency (for example `done`/`archived` after filtering), the missing edge is silently dropped and graph rendering continues without broken connectors
 - **Auto-layout**: Sugiyama-style layered layout (`computeAutoLayout`) groups nodes by dependency depth and spaces layers consistently
 - **Edge drawing**: SVG bezier curves from source bottom-center to target top-center, with arrowheads showing dependent → dependency direction
-- **Interaction**: drag-to-pan canvas, cursor-centered wheel zoom, pinch-to-zoom with stationary midpoint, keyboard shortcuts, zoom toolbar, reset, and fit-to-graph
+- **Interaction**: drag-to-pan canvas background, drag-to-reposition nodes, cursor-centered wheel zoom, pinch-to-zoom with stationary midpoint, keyboard shortcuts, zoom toolbar, reset, and fit-to-graph
 - **Fit-to-graph**: computes node bounding box with layout node dimensions and applies zoom/pan so the graph fits in viewport with padding
 - **Initial auto-fit**: when no saved scoped positions exist, the first non-empty render auto-fits once; subsequent updates preserve user navigation state
 - **Animated transitions**: fit/reset operations animate `transform` (`var(--transition-normal)`), while continuous drag/wheel/pinch stays transition-free for responsiveness