feat: configurable edge line style + width per type and per edge

Edge render (solid/dashed/dotted) and stroke width were hardcoded per
edge type. Expose both as user settings.

- Custom Style modal (Edges): line-style buttons, 1-4x width slider,
  live preview; left-list swatch renders the actual line.
- Per-edge EdgeModal: same controls; line style follows the type preset
  live until overridden.
- Renderer applies line_style/width_mult over BASE_STYLES (width scales
  markers + animation overlays); unset keeps the type default look.
- Persist line_style/width_mult through serializer, canvas save, and the
  edges API (new nullable columns, idempotent migration).

ha-relevant: yes

backend/app/db/models.py

@@ -85,6 +85,8 @@ class Edge(Base):
     speed: Mapped[str | None] = mapped_column(String)
     custom_color: Mapped[str | None] = mapped_column(String)
     path_style: Mapped[str | None] = mapped_column(String)
+    line_style: Mapped[str | None] = mapped_column(String)
+    width_mult: Mapped[float | None] = mapped_column(Float)
     animated: Mapped[str] = mapped_column(String, nullable=False, default='none')
     marker_start: Mapped[str] = mapped_column(String, nullable=False, default='none')
     marker_end: Mapped[str] = mapped_column(String, nullable=False, default='none')