feat(editor): edge selection + properties-panel edge-info + canvas-wide deselect

Three flow-editor UX upgrades:

1. Wider deselect zone. A new translucent
   Positioned.fill GestureDetector wraps the outermost
   Stack (above the InteractiveViewer). Tapping the dark
   area outside the canvas grid — which used to do
   nothing — now closes the properties panel.

2. Edge selection. Click a wire → controller.selectEdge
   stamps the edge key. Click again or click background →
   deselects. Selection is mutually exclusive with step
   selection, enforced inside the controller so the panel
   never tries to render both. Selected edges highlight
   with the same accent the hover state uses.

3. Properties panel grows an edge-info mode. When an
   edge is selected, the panel shows:
   - source qualified name (e.g. inputs.document or
     summarize.response)
   - target qualified name
   - type colour swatch + name per endpoint (LabVIEW
     palette)
   - type-compatibility pill (green / red)
   - Disconnect button that removes the edge in one click
   l10n: 8 new edgeInfo* strings in EN + DE.

Editor tests still pass (20).

Signed-off-by: flemming-it <sf@flemming.it>

pubspec.yaml

@@ -1,6 +1,6 @@
 name: fai_studio_flow_editor
 description: Swappable inline YAML editor for F∆I Studio flows.
-version: 0.12.0
+version: 0.13.0
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor