fix: resolve 4 bugs in termui

[saurabhhhcodes]

Description

This PR fixes real bugs found in the codebase:

• Removed redundant boolean comparison: x === true is equivalent to x (and x === false to !x), and shorter to read.
• Replaced global isNaN with Number.isNaN: the global version coerces its argument, so isNaN('1') returns false while Number.isNaN is strict.
• Added explicit radix to parseInt: without 10, strings like '0x1F' or '08' parse in unintended bases.
• Simplified empty-string validation: comparing trim() to '' misses whitespace-only input; .trim().length === 0 is explicit.

Type of Change

• Bug fix (non-breaking change fixing an issue)

How Has This Been Tested?

• Local manual testing

Checklist

• My code follows the style guidelines
• I have performed a self-review

Related Issue

Ref: #3425

Summary by CodeRabbit

• Bug Fixes
  • Improved input validation for calculator expressions and selection prompts.
  • Ensured widget gallery keyboard input is interpreted consistently as decimal numbers.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 19ea414d-4a98-40ce-b56e-5e0beaa3ed13

📥 Commits

Reviewing files that changed from the base of the PR and between 6c7584e and f125dcf.

📒 Files selected for processing (3)

• examples/calculator/src/index.tsx
• examples/widget-gallery/src/index.ts
• packages/ui/src/prompts.ts

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📝 Walkthrough

Walkthrough

The change updates three input-handling checks: trimmed calculator expressions use length, widget tab keys use decimal parsing, and prompt choices use strict NaN validation.

Changes

Input Handling Updates

Layer / File(s)	Summary
Calculator expression guard examples/calculator/src/index.tsx	CalculatorApp.evaluate() checks whether the trimmed expression length is zero.
Widget shortcut parsing examples/widget-gallery/src/index.ts	WidgetGalleryApp.handleKey passes radix 10 to parseInt.
Prompt choice validation packages/ui/src/prompts.ts	promptSelect uses Number.isNaN instead of global isNaN.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• Karanjot786/TermUI#3418: Modifies the same calculator and prompt validation logic.
• Karanjot786/TermUI#3413: Updates widget tab-key parsing with an explicit decimal radix.
• Karanjot786/TermUI#3367: Replaces global isNaN with Number.isNaN in promptSelect.

Suggested labels: type:bug

Suggested reviewers: karanjot786

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description explains the fixes and testing, but it omits packages, uses "Ref: #3425" instead of "Closes #3425", and leaves required fields incomplete.	Add the affected packages, use Closes #3425``, and complete the required checklist and GSSoC 2026 fields.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately identifies this pull request as a bug fix addressing four TermUI bugs.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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