Round out the canvas editing experience.
- Dragging one card of a multi-selection now moves and persists the whole
selection (React Flow hands the full dragged set to onNodeDragStop).
- A category band is draggable by its header, which moves every technology in
that band; the band body stays click-through.
- A "Snap: On/Off" toolbar toggle (default on, remembered per browser) snaps
dragged cards to a 46px grid that divides the column and row steps, so
snapped cards line up with the automatic layout.
- Connection lines are adjustable: select an edge to reveal "+" handles at each
segment's midpoint (click to add a bend), drag bends to reroute, and
double-click a bend to remove it. Routes persist per browser.
- Edges route around technologies they are not connected to, and corners are
rounded, so lines no longer cut through unrelated cards.
- Ctrl/Cmd-clicking to multi-select no longer pops the side panel over the
canvas.
- `tech-tree-layout.int.spec.ts`: pure layout (wrapping, manual overrides,
category bands, forward-edge warnings, cycles, catalog-stability).
- `tech-tree-prereq-edits.int.spec.ts`: add/remove/switch prerequisites with
cycle rejection and the pin-on-connect stability contract.
- `tech-tree-stability.int.spec.ts`: freeze pins every relocated card on a
data change, plus edge visuals (per-path colors, per-source lanes).
- `tech-categories.int.spec.ts`: CRUD, RBAC, division gating, FK set-null,
position round-trip.
- `tech-tree.e2e.spec.ts`: nav entry, render, preview/panel edits, both
quick-create paths, drag persistence + Auto-arrange, save positions
surviving a reload, connect/disconnect, and multi-column wrapping.
