fix(playback): bypass cache for live streams opened via ZoomableContentView

Live streams routed through the Live Streams tab go through ShowVideoStreaming
→ ZoomableContentView, not LiveActivity.kt. ZoomableContentView is generic
and was calling VideoView without isLiveStream=true, so live stream segments
ended up in SimpleCache. The cached HLS manifest then went stale (live
manifests roll constantly) and playback stopped after ~30 seconds.

Plumb the flag through the data model: add isLiveStream to MediaUrlVideo,
set it true when ShowVideoStreaming constructs the model, and pass it from
ZoomableContentView into VideoView. URL heuristic (.m3u8) wasn't viable —
that would also bypass cache for on-demand NIP-71 multi-rendition videos
and defeat the original feature.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/richtext/MediaContentModels.kt

@@ -97,6 +97,7 @@ open class MediaUrlVideo(
     val contentWarning: String? = null,
     mimeType: String? = null,
     thumbhash: String? = null,
+    val isLiveStream: Boolean = false,
 ) : MediaUrlContent(url, description, hash, dim, blurhash, uri, mimeType, thumbhash)
 
 @Immutable