fix(editor): port docks elegantly — line terminates at perimeter

Stefan's "the connections to the round ports look
unelegant" feedback was about three concrete things:

(1) The bezier endpoint sat at the port dot's GEOMETRIC
CENTRE. The dot was rendered ON TOP, so the line visually
disappeared into the centre of the dot — looked like a
thread being eaten by a bead.

(2) A separate arrow-head triangle was drawn on top of the
dot, doubling the visual terminator and making the
endpoint look "noisy".

(3) The dots were hollow rings the same colour as the
line, so the boundary between line and dot blurred
visually. They didn't read as connectors.

Fix:

 - EdgeSegment now carries `fromSide` + `toSide`
   (left / right) so the painter knows which way to
   shorten each endpoint. The bezier ends `portRadius` px
   short of the centre — exactly on the dot's outer
   perimeter, on the side facing the line. The line now
   "docks" cleanly at the rim.

 - The end-cap triangle arrow is gone. The dot itself is
   the visual terminator; an arrow on top was redundant.

 - Port dot redesigned as a connector-socket: outer ring
   defines the footprint, inner 4-px surface-coloured pin
   appears when connected. The result reads as "an active
   socket with a plug seated in it" rather than "a hollow
   circle next to a card edge". Empty ports stay surface-
   filled rings — unambiguous empty-slot signal.

 - Bezier handle length floor raised from 40 to 60 px so
   vertical detours (e.g. inputs-endpoint → step far below)
   curve gracefully out of the start before dropping
   instead of kinking near the origin.

 - Stroke width nudged from 1.8 to 2.0 px for the normal
   accent so the line carries the right visual weight
   against the new docked-at-perimeter port treatment.

Visual: lines now look like cables seated into sockets,
not strings vanishing under beads.

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: 0.5.1
+version: 0.5.2
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor