fix: preserve edge connection points in YAML export/import (#208)

YAML round-trip dropped every edge's handles: export never wrote them
and import hardcoded sourceHandle='bottom'/targetHandle='top-t'. After
import all connections collapsed onto slot 0 ("converge at a single
point"). Per-side handle counts were dropped too, so the extra bottom
slots did not even exist to attach to.

- yaml types: add sourceHandle/targetHandle to connections and
  top/bottom/left/rightHandles to nodes
- export: write each edge's real handles + per-side counts (only above
  the side default); orient parent-edge handles parent->child
- import: restore handle counts onto node data and use the stored
  handles, falling back to the legacy defaults for pre-existing YAML

Positions remain dagre-managed (unchanged); this restores connection
points only.

ha-relevant: yes

frontend/src/types/yaml.ts

@@ -4,6 +4,10 @@ export interface YamlNodeConnection {
   label: string
   linkType?: EdgeType
   linkLabel?: string
+  // Connection points (React Flow handle IDs) the edge attaches to. Preserved so
+  // a manual layout round-trips instead of collapsing every edge onto slot 0.
+  sourceHandle?: string
+  targetHandle?: string
 }
 
 export interface YamlNode {
@@ -23,4 +27,11 @@ export interface YamlNode {
   cpuCore?: number
   ram?: number
   disk?: number
+  // Per-side connection-point counts. Only written when a side has more than its
+  // default (top/bottom 1, left/right 0) so the referenced handle slot exists on
+  // re-import — without it React Flow falls back to slot 0.
+  topHandles?: number
+  bottomHandles?: number
+  leftHandles?: number
+  rightHandles?: number
 }