feat(editor): edge hover + edge menu + pattern picker + zoom indicator

Closes four operator complaints from the 0.5.3 review.

1. Edges now react to hover.

   Added an interaction layer between the edge painter and
   the endpoint nodes. A MouseRegion tracks the cursor in
   canvas coords; every move runs a spatial hit-test
   against each edge's bezier sampled at 24 points. The
   closest edge within 8 px highlights in the primary
   accent so the operator sees what they're aiming at.
   Cursor leaves canvas → highlight clears.

2. Right-click / long-press on edges opens a Disconnect
   menu.

   Same interaction layer carries `onSecondaryTapDown` and
   `onLongPressStart` handlers. Both run the same hit-test
   and pop the menu at the cursor. Disconnect clears the
   target's expression (step.with[field] = '' or
   outputs[name] = ''), edge disappears, target port flips
   outlined.

3. Long-press is now available alongside right-click
   EVERYWHERE the context menu lives — step nodes, port
   dots, edges. Trackpad-only users whose "two-finger
   click" isn't bound to secondary get the same affordances
   as mouse users.

4. Background pattern is now operator-selectable: cycles
   through dots → grid → blank via a small icon button on
   the bottom-right canvas controls. The button's icon and
   tooltip reflect the next state. Picked dots as the
   default because they're least visually noisy at scale.

5. Zoom indicator: a mono "100%" readout next to Fit-to-
   screen shows the current InteractiveViewer scale,
   updating live as the operator pinches / scrolls. Tap
   to snap back to 100 % without losing pan position.

The bottom-right control cluster is now:
  [pattern] [reset layout] [fit to screen] [zoom%]

Version 0.5.3 -> 0.6.0 (minor bump — new interaction
surfaces + visible toolbar additions).

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: 0.5.3
+version: 0.6.0
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor