feat(frontend): create comment feature on video page

[keanu-a]

Description

Builds on timestamp-marker PR to make comments postable. Uses can now comment from two areas:

• The video player that anchors current playback time
• Comment section for general comments with no timestamp
  Only timestamped comments render as markers on the video time range bar. general comments live in the comment section only

Changes

• Post comment from video player -> anchors timestamp -> creates marker on timeline
• Post comment from comment section -> no timestamp
• Comments are ordered from newest-first
• Multi-line comments are supported
• Made timestampSeconds nullable on the backend and changed for typings/usage on frontend
• Created usePostComment hook so reused logic for video player comment input and comment section input
• Extracted CommentInput from comment section to handle its own state and not inflate VideoPageClient

Deferred

• Login prompt when commenting
• Sorting comments
• Comment likes and replies
• Real profile pictures