feat: extract and display meeting participants

[lucasheriques]

Summary

• Extract unique participants from transcript segments when recording ends
• Display participant count and names in RecordingView header
• Show participants as blue badges below meeting metadata

Implementation

Client-side extraction (recordingService.ts):

• When meeting ends, extract unique participants from all transcript segments
• Single-pass array operation with Set deduplication (efficient even for 1000+ segments)
• Update recording with participants list via API

UI Display (RecordingView.tsx):

• Show participant count in header metadata ("3 participants")
• Display participant names as blue badges
• For active recordings: extract from current segments
• For past recordings: use stored participants field

Notes

Builds on #74 (live transcripts) which provides the RecordingView component.

[lucasheriques]

• feat: extract and display meeting participants #75 👈 (View in Graphite)
• feat: live transcript view with split panel UI #73: 1 other dependent PR (#76)
• feat: integrate Recall SDK for meeting detection and recording #65: 1 other dependent PR (#67)
• main

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