fix(interactions): ignore secondary-button cell range drags

[ghiscoding]

replicate Slickgrid-Universal PR 2756 into SlickGrid. You can see the included ChatGPT summary below.

## Summary

Fixes unintended cell-range drags triggered by secondary mouse-button actions.

## Why

Right-clicking or using another secondary mouse button on a selected cell could unexpectedly start a drag operation and alter the selection.

## Changes

• Ignore non-primary mousedown events in Draggable.
• Preserve compatibility with synthetic mousedown CustomEvent usage.
• Add Cypress regression coverage for secondary-button cell drags.

## Validation

• All Cypress tests passed.
• npm run lint
• npm run build:types
• npm run build:prod
• git diff --check

## Comments

This is the core SlickGrid equivalent of the upstream Universal fix:
ghiscoding/slickgrid-universal#2756

No documentation update is required because this is an internal behavior fix.

## AI / LLM assistance

• AI / LLM assistance used:
  • No
  • Yes
• If Yes:
  • which tool/model: OpenAI Codex / GPT-5.6 Luna
  • how was it used: Reviewed the upstream fix, implemented the core SlickGrid change, added regression coverage, and validated the build and tests.

## Checklist

• The changes are limited to only one scope.
• Tests were added or updated where appropriate.
• Documentation was updated where appropriate.