Draft: Add progress support

[Explosion-Scratch]

Draft for adding progress support and abort signal, handlers get passed in doConvert. New param for doConvert is ConvertContext:

/** Context passed to doConvert for progress/log reporting. */
export interface ConvertContext {
  /**
   * Report progress.
   * @param message - Status message to display.
   * @param value - Absolute 0–1 value, or updater `(old) => new`.
   */
  progress: (message: string, value: number | ((prev: number) => number)) => void;
  /**
   * Append a timestamped log entry.
   * @param message - The log message.
   * @param level - Log level ('log', 'error', 'debug', 'warn'). Defaults to 'log'.
   */
  log: (message: string, level?: LogLevel) => void;
  /** AbortSignal for cooperative cancellation. */
  signal: AbortSignal;
  /** Throws if the signal has been aborted. Call between async steps. */
  throwIfAborted: () => void;
}

This allows in doConvert to for instance at convenient times ctx.throwIfAborted() or ctx.progress("Transcoding...", 0.5)

[Explosion-Scratch]

Note I've continued from #580

[p2r3]

Closing, see #580

[Explosion-Scratch]

But what about the progress functionality only as opposed to the UI tweaks - this is a lot of under the hood stuff.

[Explosion-Scratch]

This allows progress and logs via the doConvert method which now exposes a ctx param - I think this is a good approach regardless of the frontend but once #73 is merged (if it is) then I will re-PR with the backend changes to integrate with that frontend.