fix(editor): draft line reaches cursor + ports react on hover

Two operator-visible fixes from the 0.5.2 review:

(1) Draft line stopped short of the cursor. EdgePainter
shortens every segment endpoint by portRadius to dock at
port-dot perimeters — but the DRAFT segment's `to` is the
cursor itself, not a port. Result: while dragging a
connection, the live line ended 6 px short of where the
operator's mouse actually was. Felt like the cable
wouldn't reach.

EdgeSegment gains `shortenFrom` / `shortenTo` flags
(default true). The draft segment passes `shortenTo:
false`, so the line tip tracks the cursor exactly. From-
side stays shortened because the FROM is still a real
port dot.

(2) Ports had no hover affordance — mouse-over was
invisible. Operators couldn't tell ahead of time that a
port was interactive.

Adds `_hoveredPort` state on the canvas, updated via the
existing MouseRegion's onEnter / onExit. Port dot now has
three focus levels:

  resting → 12 px, 1.8 px border, no glow
  hover   → 15 px, 2.2 px border, soft accent glow
  drop-target (drag is over it) → 18 px, 2.5 px border,
            stronger glow

The hover state pre-shadows the drop-target state —
operators see "yes, this port is grabbable / droppable"
before they commit to dragging. When the drag does start,
the drop-target halo is the same family of treatment,
just stronger, so the visual progression reads as one
continuous interaction.

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: 0.5.2
+version: 0.5.3
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor