feat(handles): allow 1-48 bottom connection points via slider

Replaces the 4-option dropdown with a 1..48 slider. Existing handle IDs
('bottom', 'bottom-2'..'bottom-4') are preserved and the hand-tuned pixel
positions for counts 1..4 are locked in (regression-tested) so saved
canvases render identically. Counts >=5 use uniform spacing; node card
auto-grows in width so handles stay clickable. Existing edge-remap on
shrink scales unchanged thanks to its index-based loop.

ha-relevant: yes

frontend/src/components/canvas/__tests__/BaseNode.test.tsx

@@ -56,8 +56,9 @@ vi.mock('@/utils/propertyIcons', () => ({
 }))
 
 vi.mock('@/utils/handleUtils', () => ({
-  BOTTOM_HANDLE_IDS: ['bottom'],
-  BOTTOM_HANDLE_POSITIONS: { 1: [50] },
+  bottomHandleId: (idx: number) => idx === 0 ? 'bottom' : `bottom-${idx + 1}`,
+  bottomHandlePositions: () => [50],
+  clampBottomHandles: (n: unknown) => typeof n === 'number' ? n : 1,
 }))
 
 beforeEach(() => { mockZoom = 1 })