feat(frontend): show estimated end date/time preview in CreateStreamF…

[Danielodingz]

Closes #163

[wave4][frontend] Show estimated end date/time preview in CreateStreamForm

Summary

As the user fills in the Duration (hours) and Start In (minutes) fields, the form now computes and displays a real-time estimated end date/time preview directly below the duration input.

Changes

File: frontend/src/components/CreateStreamForm.tsx

• Added the missing durationHours input field (existed in state and validation but was not rendered in the JSX).
• Added an estimatedEndLabel computed value (inline IIFE, no hooks) that derives the estimated end timestamp from startAt + durationSeconds.
• Rendered the preview as a <span className="field-hint"> with aria-live="polite" below the duration input — clears automatically when either field is empty or invalid.

Acceptance Criteria

• Derived from startAt + durationSeconds in real time
• Format: Ends: Apr 30, 2026 at 14:32 UTC
• Updates on every keystroke without debounce
• Clears when either field is empty/invalid
• Uses Intl.DateTimeFormat — no external date library

Notes

• No new dependencies introduced.
• No existing logic, formatting, or unrelated code modified.
• startAt derivation mirrors the existing logic in handleSubmit exactly.

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[Danielodingz]

Hey @ritik4ever please check PR and merge, Thanks