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.
• Prevented interval leak: repeated mounts now clear the previous interval before scheduling a new one.
• 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: #3563

Summary by CodeRabbit

• Bug Fixes

  • Improved calculator validation for negative-number inputs.
  • Prevented duplicate weather refresh timers when refreshing weather data.

• Refactor

  • Improved paragraph detection in the chat app without changing its visible behavior.

[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: 850d2b9e-bab3-491b-854e-f6249ad9b7a5

📥 Commits

Reviewing files that changed from the base of the PR and between 6c7584e and 2701ba2.

📒 Files selected for processing (3)

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

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📝 Walkthrough

Walkthrough

The changes update three examples: strict Number.isNaN validation in the calculator, length-based blank-line detection in the chat app, and managed refresh interval replacement in the weather app.

Changes

Calculator validation

Layer / File(s)	Summary
Strict negative-number validation examples/calculator/src/index.tsx	safeEval uses Number.isNaN for initial negative-number validation.

Chat parsing

Layer / File(s)	Summary
Blank-line detection examples/chat-app/src/index.tsx	parseBlocks uses the trimmed line length to detect empty paragraph lines.

Weather refresh lifecycle

Layer / File(s)	Summary
Refresh interval management examples/weather/src/index.tsx	The refresh logic clears window.__interval before storing a new 5-second interval handle.

Estimated code review effort: 2 (Simple) | ~5 minutes

Possibly related PRs

• Karanjot786/TermUI#3421: Overlaps with the chat parsing and weather interval changes.
• Karanjot786/TermUI#3454: Modifies the same calculator safeEval validation.
• Karanjot786/TermUI#3553: Modifies the same weather interval cleanup.

Suggested reviewers: karanjot786, rosheshchaware

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description explains the bug fixes and testing, but it omits the package section, GSSoC details, and the required issue-closing syntax.	Add the affected package(s), use Closes #3563``, and complete the applicable checklist and GSSoC fields.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses the required format and clearly identifies the PR as a bug fix covering 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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