Add snap preview for point placement mode

[AnalysisRelevant]

This is my first open source contribution, so I appreciate any feedback on the implementation.

Fixes #719 — when placing a point, there was no visual preview showing where the point would land before clicking, making snapping to ways and nodes unintuitive.

Changes:

• Added modules/core/SnapSystem.js — a utility that computes snap targets (ways, nodes, free placement) from pointer event data. Extracted as a reusable class so other modes could potentially use it.

• Modified AddPointMode.js to show a live preview node that follows the cursor and snaps to nearby features before the user clicks. The preview is created in staging and removed via editor.revert() on click or cancel.

• Modified DrawBehavior.js to run snap detection on move and click events, attaching the result as eventData.snapResult. The original eventData fields are left unmodified so existing behavior in DrawLineMode and other consumers is unaffected.

The preview node is marked with the drawing class so the Pixi renderer treats it as non-interactive, preventing it from intercepting hit tests on underlying ways.

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