refactor(feeds): push content warning into ZoomableContentView per media

Move the blurhash-aware sensitivity warning from feed card wrappers into
ZoomableContentView so every media item shows its own warning sized to
its own aspect ratio over its own blurhash. Grid posts now warn per
image instead of once over the whole grid.

- Add isSensitive to MediaUrl{Image,Video} (defaults to contentWarning != null)
- Populate isSensitive and contentWarning when building media in the
  picture, video, and file-header feeds plus their display variants
- Drop the outer SensitivityWarning wrappers now handled inside
  ZoomableContentView via SensitivityWarningOverBlurhash

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

@@ -51,6 +51,7 @@ open class MediaUrlImage(
     dim: DimensionTag? = null,
     uri: String? = null,
     val contentWarning: String? = null,
+    val isSensitive: Boolean = contentWarning != null,
     mimeType: String? = null,
 ) : MediaUrlContent(url, description, hash, dim, blurhash, uri, mimeType)
 
@@ -62,11 +63,12 @@ class EncryptedMediaUrlImage(
     dim: DimensionTag? = null,
     uri: String? = null,
     contentWarning: String? = null,
+    isSensitive: Boolean = contentWarning != null,
     mimeType: String? = null,
     val encryptionAlgo: String,
     val encryptionKey: ByteArray,
     val encryptionNonce: ByteArray,
-) : MediaUrlImage(url, description, hash, blurhash, dim, uri, contentWarning, mimeType)
+) : MediaUrlImage(url, description, hash, blurhash, dim, uri, contentWarning, isSensitive, mimeType)
 
 @Immutable
 open class MediaUrlVideo(
@@ -79,6 +81,7 @@ open class MediaUrlVideo(
     val authorName: String? = null,
     blurhash: String? = null,
     val contentWarning: String? = null,
+    val isSensitive: Boolean = contentWarning != null,
     mimeType: String? = null,
 ) : MediaUrlContent(url, description, hash, dim, blurhash, uri, mimeType)
 
@@ -93,11 +96,12 @@ class EncryptedMediaUrlVideo(
     authorName: String? = null,
     blurhash: String? = null,
     contentWarning: String? = null,
+    isSensitive: Boolean = contentWarning != null,
     mimeType: String? = null,
     val encryptionAlgo: String,
     val encryptionKey: ByteArray,
     val encryptionNonce: ByteArray,
-) : MediaUrlVideo(url, description, hash, dim, uri, artworkUri, authorName, blurhash, contentWarning, mimeType)
+) : MediaUrlVideo(url, description, hash, dim, uri, artworkUri, authorName, blurhash, contentWarning, isSensitive, mimeType)
 
 @Immutable
 abstract class MediaPreloadedContent(