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/schemas/edges.py

@@ -14,6 +14,8 @@ class EdgeBase(BaseModel):
     speed: str | None = None
     custom_color: str | None = None
     path_style: str | None = None
+    line_style: str | None = None
+    width_mult: float | None = None
     animated: str = 'none'
     marker_start: str = 'none'
     marker_end: str = 'none'
@@ -43,6 +45,8 @@ class EdgeUpdate(BaseModel):
     speed: str | None = None
     custom_color: str | None = None
     path_style: str | None = None
+    line_style: str | None = None
+    width_mult: float | None = None
     animated: str | None = None
     marker_start: str | None = None
     marker_end: str | None = None