Consolidate now-playing.txt setup into folder selection

[paulpv]

Problem

• Configuring now-playing.txt currently lives under Settings → Configuration, separate from the media folder picker.
• Users who just choose a media folder via "Choose folder..." can miss the extra configuration step, leaving the integration with overlays/widgets incomplete.
• Maintaining two different dialogs for related permissions increases friction and chances of mismatched handles.

Proposal

• Fold the now-playing.txt handle request into the existing "Choose folder..." flow so users can approve both permissions in one pass.
• Default the file location to live alongside the selected media folder (with the option to adjust if needed) to minimize extra prompts.
• If a now-playing.txt handle already exists, surface it during the folder selection so users can confirm or replace it.

Acceptance Criteria

• Choosing a media folder guides the user through setting up or confirming the now-playing.txt handle without navigating to Settings.
• Returning users who already configured the file are shown the existing handle and can keep or change it within the same flow.
• The combined flow avoids redundant permission prompts and works in supported Chromium-based browsers.

Open Questions

• Should the default location always be inside the chosen media folder, or should the user be encouraged to pick any writable path?
• How should we message users about overwriting an existing now-playing.txt file if they move it?