feat(editor): five distinct type colours + wired/total badge in step header

Two operator-visible improvements:

 - Five payload types now get five distinct hues that
   survive both light and dark themes:
     text   → blue (0xFF42A5F5)
     bytes  → deep orange (0xFFFF7043)
     json   → purple (0xFFAB47BC)
     file   → green (0xFF66BB6A)
     number → amber (0xFFFFCA28)
   Previously bytes + file shared a colour and the palette
   collapsed under custom theme plugins. Hardcoded hues
   keep them readable everywhere.

 - Step headers now carry a small "wired/total" badge.
   When all with-fields carry a `$src.field` reference,
   the badge collapses to a green check icon — the visual
   "this module is fully wired" signal Stefan asked for.
   Partial-wiring shows e.g. "3/5" in a pill matching the
   step accent. Endpoint nodes don't render the badge
   (ratio doesn't apply there).

Computed live from graph.edges + step.with_ values, so the
indicator updates the moment an operator drags a
connection in or out.

Version 0.5.0 -> 0.5.1.

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: 0.5.0
+version: 0.5.1
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor