feat: show liquidation price in margin modal

[380712766-dot]

Summary

• Show the current liquidation price in the edit margin modal.
• Update the new liquidation price calculation when adding or removing margin.
• Use the market liquidation threshold from marketInfos so the modal matches the position liquidation price logic.
• Avoid showing infinite leverage when removing all available margin.

Tests

• Checked the changed Svelte file for balanced script tags.
• Ran git diff --check.

Note: I could not run the full npm build locally because npm is not installed in this environment.

Closes #4

[380712766-dot]

I opened a PR for this issue: #257. Could you please assign this bounty to me?