Add slides filter to conference schedule

[Debagithub567]

Closes #2420

Description

This PR adds support for filtering conference sessions based on whether presentation slides are attached.

Changes

• Added a hasSlides property to ScheduleSession
• Derived the hasSlides value from the existing session.files data
• Added a Slides filter to the conference schedule
• Reused the existing generic filtering mechanism without introducing additional filtering logic

Testing

• Verified that the Slides filter appears in the conference schedule
• Verified that sessions with attached files appear under Yes
• Verified that sessions without attached files appear under No
• Confirmed that the filter works alongside the existing schedule filters

Screenshot

The new Slides filter with Yes/No options in the conference schedule.

[linux-foundation-easycla]

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• ✅ login: Debagithub567 / name: Debajit Roul (907e8f0)

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[martinbonnin]

Thank you!