feat: update Player buttons UI

[santosral]

🚀 Summary

Updates the Learning Unit page's content-action buttons to better reflect what they trigger and to balance their visual weight. Renames the buttons to match their underlying content types ("Podcast"/"Video" instead of the verb-based "Listen"/"Watch"), swaps the Video button's icon to the more recognisable monitor-with-play, and adjusts variant hierarchy so neither action visually dominates when both content types are available on a unit.

✏️ Changes

• Renamed the "Listen" button → "Podcast" across all three podcast play states (initial play, resume, resume-from-checkpoint). The "Pause" label is intentionally unchanged.
• Renamed the "Watch" button → "Video".
• Replaced the lucide Video icon on the Video button with MonitorPlay (monitor outline with a play triangle).
• Added a hasPodcastAndVideo derived state on +page.svelte. When both content types are present on a unit, both buttons render as secondary so neither dominates; when only one type is present, that single CTA renders as primary.
• File touched: src/routes/(main)/(protected)/unit/[id]/+page.svelte (only).

[nicholasjjlim]

LGTM 👍