feat(editor): unified port model — one dot per port, sided + typed + state-aware

Rewrites the port-rendering model around the operator's
expectation that each port is a single visible dot with
clear state. Closes four concrete complaints from the
0.4.0 review.

Removes the redundant inline dot:
 - FlowNode._body no longer paints any inline 8-px dot.
   The canvas-side dot is the SINGLE visible port. Size
   matches the label row (12-px diameter, NodeGeometry.
   portDotSize). No more "two dots per port" doubling.

Sides + alignment:
 - New NodePortSide enum on FlowNode. `left` for step
   inputs + outputs endpoint inputs; `right` for the
   inputs endpoint (its body labels represent OUTPUTS of
   the node — data flows OUT to downstream steps, so the
   port dots belong on the right edge).
 - Body labels right-align on right-side nodes,
   left-align elsewhere. The 18-px port-gutter inside the
   body keeps the dot from overlapping label text on the
   port-bearing edge.
 - Inputs endpoint dots now hang off the RIGHT edge of
   the node, matching where the visible label gravitates.
   Previously the dot was on the right but the labels
   crowded the left — visually disconnected.

State-aware rendering:
 - Port dots render FILLED when participating in an
   edge, OUTLINED when dangling. `_connectedPorts` walks
   graph.edges once per build and marks every endpoint.
   Operator sees at a glance which inputs are wired vs
   which need attention.
 - Type-coloured: inputs endpoint dots take the type
   accent (text → primary, bytes/file → tertiary,
   json → secondary, number → amber, unknown → muted).

Edge deletion via port context menu:
 - Right-click on a wired input port (step or outputs
   endpoint) opens a Disconnect popup. Confirming clears
   the with-field / output expression to empty, edge
   disappears, port flips outlined. No need to chase the
   YAML buffer manually.
 - Unwired ports get no context menu (nothing to
   disconnect) — keeps the menu honest.

The drop-target halo on drag stays — closest input port
within snap distance still inflates to 18 px and gets a
brighter halo, so the connection-drawing UX is unchanged.

Version 0.4.0 -> 0.5.0 — refactored API + visible port
model change warrants the minor bump.

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: 0.4.0
+version: 0.5.0
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor