fix(memory-graph): enable touch and mobile node dragging and interaction

[Adityakk9031]

Problem

In packages/memory-graph/src/canvas/input-handler.ts, desktop mouse interaction (onMouseDown) tests the spatial index (this.spatialIndex.queryPoint) to detect when a node is clicked or dragged.

However, touch interaction (onTouchStart) unconditionally initiated a canvas pan (this.isPanning = true) for single touches without querying the spatial index for a node hit. Consequently:

• On smartphones, tablets, iPads, and touch-screen laptops, graph nodes were completely undraggable.
• Attempting to drag or reposition a node on touch screens panned the entire canvas viewport rather than dragging the selected node.

Solution

1. Updated onTouchStart to query this.spatialIndex.queryPoint(startWorld.x, startWorld.y) before defaulting to panning. If a node is hit, it sets this.draggingNode = node and invokes onNodeDragStart(node.id, node) while disabling viewport panning.
2. Updated onTouchMove to reposition the active draggingNode (fx, fy, x, y) and request canvas re-renders when a node is being dragged on touch.
3. Updated onTouchEnd to clear fixed drag coordinates (fx/fy), reset draggingNode, and fire onNodeDragEnd().
4. Recorded touchStartNodeId at touch start so tap-to-select cleanly triggers onNodeClick upon tap release without interference from sub-threshold finger jitter.
5. Added unit tests in packages/memory-graph/src/__tests__/input-handler-touch.test.ts verifying node dragging on touch, viewport panning on empty space, and tap selection.

Verification

Ran bun test packages/memory-graph/src/__tests__/input-handler-touch.test.ts (all 8 tests passed).

[Adityakk9031]

Hey @Dhravya @MaheshtheDev, could you please take a look at this fix when you have a moment? Thank you!