feat(editor): port geometry fix + type colours + endpoint editor + unsaved badge

Four operator-visible improvements in one cut.

Port geometry (the "connection dots don't line up with the
labels" bug):
 - Header now has a fixed two-line structure (title + 16-px
   subtitle slot) regardless of node kind, so the body's
   port-row Y-offsets are identical across step nodes and
   endpoint nodes.
 - NodeGeometry constants rewritten so the inline 8-px dot
   inside each port row and the 16-px canvas-side dot both
   compute to the same Y. Port rows now anchor where the
   eye expects them.

Port type colours:
 - Inputs endpoint labels carry `name: type` (e.g.
   `doc: bytes`), so the FlowNode body parses the type and
   tints the inline dot per type: text → primary,
   bytes/file → tertiary, json → secondary, number → amber,
   unknown → muted. Step input ports stay muted until a
   future commit can read module manifests for type info.

Endpoint editor:
 - Inputs and outputs nodes are now selectable. Selecting
   opens a dedicated editor in the properties panel:
   add / rename / retype / remove entries graphically
   instead of forcing the operator into the text tab.
   Inputs editor offers a Type dropdown
   (text / bytes / json / file / number);
   outputs editor exposes name + expression
   (e.g. `$step.field`).

Unsaved badge:
 - The toolbar's tiny 8-px dirty dot was easy to miss.
   Replace it with a coloured "unsaved" / "ungespeichert"
   chip + colour the file name itself in the primary accent
   when dirty. Operators see at a glance that a flow has
   unsaved changes, which matters because the Discard
   confirmation prompt won't fire if they don't realise
   they made changes.

Version 0.3.0 -> 0.4.0 — first new editor feature surface
since 0.3.

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: 0.3.0
+version: 0.4.0
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor