feat(calculator): Implement 'multiply' functionality

[DimaSerikov]

Summary (required)

Enable multiplication (*) operation in the calculator.

Ticket / Context (required)

• Ticket: CALC-3
• Business goal: Allow users to multiply numbers using the calculator UI.
• Related links: N/A

Type of change (required)

• Feature
• Bugfix
• Refactor
• Tech debt
• Hotfix

Scope (required)

• Backend
• Frontend
• Mobile
• Infrastructure
• Database

How to test (required)

1. Open index.html in browser.
2. Enter 2, press *, enter 3, press =.
3. Verify result is 6.

Expected result (required)

Multiplication button is enabled and expression a * b returns correct result.

Risk & impact (required)

• Risk level: Low
• User impact: Visible (new operation works)
• Backward compatibility: Yes
• Rollback plan: Revert this PR

Checklist (required)

• Self-review completed
• Tests added/updated or N/A (N/A – no test framework in this repo)
• No secrets in code
• Documentation updated or N/A (N/A)
• Logs / monitoring updated or N/A (N/A)