🎨 Palette: Improve screen reader a11y for custom buttons

[LeanBitLab]

💡 What: Added explicit Role.Button semantic roles to custom clickable Compose components in VideoPage.kt, and removed a redundant contentDescription on the expand icon.
🎯 Why: Screen readers would previously read custom buttons simply as text or generic clickable zones. The expand icon also duplicated the "Show more"/"Show less" text right next to it, making navigation tedious.
📸 Before/After: N/A (Visual UI remains unchanged)
♿ Accessibility: Ensures correct interactive component announcements by TalkBack/VoiceOver and removes confusing redundant announcements.

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PR created automatically by Jules for task 6178877680850775327 started by @LeanBitLab

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