fix: resolve 4 bugs in termui

[saurabhhhcodes]

Description

This PR fixes real bugs found in the codebase:

• Simplified empty-string validation: comparing trim() to '' misses whitespace-only input; .trim().length === 0 is explicit.
• 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.
• Replaced global isNaN with Number.isNaN: the global version coerces its argument, so isNaN('1') returns false while Number.isNaN is strict.

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: #3504

Summary by CodeRabbit

• Bug Fixes
  • Fixed calculator's handling of negative number validation for improved accuracy
  • Enhanced chat app's paragraph parsing to properly recognize whitespace-only lines as empty paragraphs
  • Improved showcase example's tab-selection key parsing with explicit base-10 number conversion

[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: cb8a74e3-20b9-4b3a-8af8-50547047ee4a

📥 Commits

Reviewing files that changed from the base of the PR and between 6c7584e and 5928f0f.

📒 Files selected for processing (3)

• examples/calculator/src/index.tsx
• examples/chat-app/src/index.tsx
• examples/showcase/src/index.tsx

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📝 Walkthrough

Walkthrough

Three example applications receive small input-handling corrections for numeric validation, whitespace-only paragraphs, and decimal tab-key parsing.

Changes

Calculator numeric validation

Layer / File(s)	Summary
Safe evaluation validation examples/calculator/src/index.tsx	safeEval converts the negative-number token before applying Number.isNaN.

Chat whitespace parsing

Layer / File(s)	Summary
Whitespace-only paragraph detection examples/chat-app/src/index.tsx	parseBlocks treats whitespace-only lines as empty paragraphs.

Showcase tab key parsing

Layer / File(s)	Summary
Decimal tab key parsing examples/showcase/src/index.tsx	handleKey parses tab-selection keys with radix 10.

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

Possibly related PRs

• Karanjot786/TermUI#3488: Modifies the same parseBlocks whitespace handling.
• Karanjot786/TermUI#3495: Replaces global isNaN with Number.isNaN for numeric validation.
• Karanjot786/TermUI#3503: Modifies the same chat-app whitespace handling.

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 required package and GSSoC sections and does not use the required issue-closing syntax.	Add the package section, complete the GSSoC section, and replace Ref: #3504 with `Closes `#3504; complete any applicable template checklist items.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the pull request as a bug fix for 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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