feat: customizable link color per edge

- Add custom_color field to EdgeData type, Edge DB model and all schemas
- HomelableEdge applies custom_color as stroke override (before selected highlight)
- EDGE_DEFAULT_COLORS extracted to utils/edgeColors.ts (react-refresh compliant)
- EdgeModal: color picker row showing effective color (custom or type default)
  with hex value, Reset button when custom color is active
- Auto-migration adds custom_color column to edges table

backend/app/db/models.py

@@ -54,6 +54,7 @@ class Edge(Base):
     label: Mapped[str | None] = mapped_column(String)
     vlan_id: Mapped[int | None] = mapped_column(Integer)
     speed: Mapped[str | None] = mapped_column(String)
+    custom_color: Mapped[str | None] = mapped_column(String)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now)