[Bug]: Hardcoded white text in meeting_insights_screen makes transcript invisible in light mode

## Description

In `lib/screens/meetings/meeting_insights_screen.dart` (lines 341-348), the `_buildTranscriptView()` method uses hardcoded `Colors.white` for speaker name and `Colors.white70` for transcript text. In light mode, the background resolves to pure white (`Colors.white`), making the text invisible and unreadable.

## Expected behavior

Text colors should be theme-aware using `Theme.of(context).colorScheme.onSurface` and `Theme.of(context).colorScheme.onSurfaceVariant`.

## To Reproduce

1. Set the app to light mode
2. Navigate to a meeting that has transcript data
3. Observe that speaker names and transcript text are invisible against the white background

## Fix

Replace `Colors.white` with `Theme.of(context).colorScheme.onSurface` and `Colors.white70` with `Theme.of(context).colorScheme.onSurfaceVariant`.