fix: timebar seeking snaps back after next episode on desktop

[prneut]

Summary

Fixed a bug where manual timeline seeking ("scrubbing") via the progress bar stops working and visually snaps back after loading a "Next Episode".

PR type

• Reproducible bug fix
• UI glitch/bug fix
• Behavior bug/regression fix
• Small maintenance only, with no UI or behavior change
• Docs accuracy fix
• Translation/localization only
• Approved larger or directional change

Why

When an episode transitions, the player state clears its playerController reference to prepare for the new video. On Desktop, the PlatformPlayerSurface reuses the native window but was only broadcasting onControllerReady(controller) during its initial launch. This left the UI with a null controller for all subsequent episodes, causing any seek actions to be silently ignored.

Desktop scope

This fix is completely scoped to the Desktop implementation (PlayerEngine.desktop.kt), specifically ensuring the onControllerReady event fires every time a new stream is attached, rather than just once.

Issue or approval

Approved in #87

UI / behavior impact

• No UI change
• No behavior change
• UI changed only to fix a documented glitch/bug
• Behavior changed only to fix a documented bug/regression
• UI change has explicit maintainer approval
• Behavior change has explicit maintainer approval

Policy check

• I have read and understood CONTRIBUTING.md.
• This PR is small, focused, and limited to one problem.
• This PR is scoped to the desktop app, desktop packaging, desktop documentation, or shared code required for desktop behavior.
• This PR is not cosmetic-only.
• Any UI change fixes a linked glitch/bug and includes visual proof, or this PR has no UI change.
• Any behavior change fixes a linked bug/regression or has explicit approval, or this PR has no behavior change.
• This PR does not bundle unrelated refactors, cleanups, formatting, or drive-by changes.
• This PR does not add dependencies, architecture changes, migrations, or product-direction changes without explicit approval.
• I listed the testing performed below.

Scope boundaries

Intentionally limited to restoring the onControllerReady callback in the stream attachment loop so the UI receives the valid controller reference.

Testing

Tested on Windows Desktop. Verified that skipping to the next episode and then clicking the time bar correctly scrubs the video instead of snapping back.

Screenshots / Video

Not a UI change.

Breaking changes

None.

Linked issues

Fixes #87