Refresh README screenshots for updated Guide UI

[rroy676]

Goal

Refresh the README screenshots so they match the current Stationarr UI after the recent branding, large-playlist editor, and Guide updates.

Problem

The README screenshots may now be outdated, especially the TV Guide screenshots, because the Guide UI now uses a two-bar layout with global app controls in the top bar and Guide-specific controls in a second controls bar.

Requirements

- Review the existing README screenshots under docs/screenshots/.
- Replace outdated TV Guide screenshots with new screenshots showing the updated two-bar Guide UI.
- If the Channel Editor screenshots are outdated because of server-side pagination/page-size controls, refresh those too.
- If the dashboard screenshots still look accurate, leave them unchanged.
- Keep both dark and light screenshots where the README currently shows both.
- Preserve the existing README image paths where possible so README links do not need major changes.

Current README screenshot paths

docs/screenshots/dashboard.png
docs/screenshots/dashboard-light.png
docs/screenshots/editor.png
docs/screenshots/editor-light.png
docs/screenshots/guide.png
docs/screenshots/guide-light.png

Validation

- README renders correctly on GitHub.
- Guide screenshots show the current two-bar Guide UI.
- Screenshot files are not unnecessarily large.
- Existing README image paths remain valid.

Related

• Related to Update README to document large-playlist editor and Guide performance changes #65.
• Related to PR feat(guide): server-side paginated guide loading with filtering and UI controls #62 Guide UI changes.
• Related to PR Phase 1: improve large-playlist editor performance #61 channel editor pagination changes.
• Related to PR Follow-up: switch README hero + favicon to branding package assets #64 branding updates.