feat: selectable marker shapes per edge endpoint

Replace the on/off arrowhead toggle with a per-end shape picker. Each end
(start/end) independently selects: none, arrow, arrow-open, circle, diamond,
or square. Markers still recolor live from the resolved stroke color.

Frontend:
- MarkerShape type + edgeMarkers util (normalizeMarker, MARKER_GEOMETRY);
  legacy boolean coerces to 'arrow' on read.
- Per-shape <marker> inner geometry; symmetric shapes use fixed orient.
- MarkerShapePicker reused in EdgeModal and CustomStyleModal.
- Serializer normalizes to shape strings.

Backend:
- Edge marker columns Boolean -> String (default 'none'); TEXT migration.
- normalize_marker() + validators coerce legacy bool / unknown values.

ha-relevant: yes

frontend/src/stores/__tests__/themeStore.test.ts

@@ -39,7 +39,7 @@ describe('themeStore', () => {
   it('setCustomStyle replaces the entire definition', () => {
     const def: CustomStyleDef = {
       nodes: { server: { borderColor: '#ff0000', borderOpacity: 1, bgColor: '#000000', bgOpacity: 1, iconColor: '#ff0000', iconOpacity: 1, width: 200, height: 80 } },
-      edges: { ethernet: { color: '#00ff00', opacity: 0.8, pathStyle: 'bezier', animated: 'none', arrowStart: false, arrowEnd: false } },
+      edges: { ethernet: { color: '#00ff00', opacity: 0.8, pathStyle: 'bezier', animated: 'none', arrowStart: 'none', arrowEnd: 'none' } },
     }
     useThemeStore.getState().setCustomStyle(def)
     expect(useThemeStore.getState().customStyle.nodes.server?.borderColor).toBe('#ff0000')