Add scientific calculator operations and dark/light theme toggle

[Copilot]

Extends basic arithmetic calculator with scientific operations (sin, cos, tan, sqrt, log, power) and implements dark/light theme switching.

Backend Changes

• Added 6 scientific operations to api/controller.js
• Implemented single-operand operation handling (sin/cos/tan/sqrt/log don't require operand2)
• Validated operation types before parsing operands

var singleOperandOps = ['sin', 'cos', 'tan', 'sqrt', 'log'];
var requiresTwoOperands = singleOperandOps.indexOf(req.query.operation) === -1;

if (requiresTwoOperands) {
  // validate and use operand2
  res.json({ result: operation(req.query.operand1, req.query.operand2) });
} else {
  res.json({ result: operation(req.query.operand1) });
}

Frontend Changes

• Added 6 scientific function buttons in purple (#6f42c1) to distinguish from arithmetic operators
• Implemented scientificOperation() for immediate single-operand calculations
• Added theme toggle button (🌙/☀️) with toggleTheme() function
• CSS variables enable smooth theme switching via .light-theme/.dark-theme body classes

Testing

Added 8 tests covering all scientific operations. All 32 tests pass.

Screenshots

Before:

After - Light Mode:

After - Dark Mode:

Original prompt

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This section details on the original issue you should resolve

<issue_title>Jan2026- Turn this calculator into a scientific calculator</issue_title>
<issue_description>Summary:

Turn this calculator into a scientific calculator and add dark and light mode.

Acceptance Criteria:

• Screenshots of before and after.
• Calculator has scientific operations now like sin and cos.
• The calculator UI is clean and modern
• The operation buttons all work
• A toggle to switch between dark and light theme.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Jan2026- Turn this calculator into a scientific calculator #24

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