Replace raw echo with output helpers in `general/convert-video`

[credfeto]

Summary

This script still uses raw echo output and should be updated to use the standard output helpers (die, info, success) defined by project shell-script rules.

Script

• general/convert-video

Why this matters

Using consistent output helpers improves readability, error reporting consistency, and rule compliance across scripts.

Acceptance Criteria

• Replace user-facing raw echo usage with die / info / success where appropriate
• Preserve script behaviour
• Pass shell linting and repository checks