[Chat] Centralize conversation report-subject resolution (#10754)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/dms/util.ts

@@ -10,6 +10,7 @@ import {EMOJI_REACTION_LIMIT} from '#/lib/constants'
 import {logger} from '#/logger'
 import {type Shadow} from '#/state/cache/profile-shadow'
 import {type ConvoState, ConvoStatus} from '#/state/messages/convo/types'
+import {type ReportSubject} from '#/components/moderation/ReportDialog/types'
 import * as bsky from '#/types/bsky'
 
 export const MESSAGE_GAP_THRESHOLD_MS = 60 * 60 * 1000
@@ -240,3 +241,41 @@ export function parseConvoView(
     return null
   }
 }
+
+/**
+ * Resolves the report subject for a conversation-level "Report conversation"
+ * action (as opposed to reporting an individual message, which always reports
+ * that message + its sender).
+ *
+ * - group: always report the whole convo, targeting the owner. Returns null if
+ *   the owner has left, in which case there is nothing to report against.
+ * - direct: report the last reportable message if there is one (i.e. the last
+ *   message exists and wasn't sent by us), otherwise report the whole convo
+ *   targeting the other user.
+ */
+export function getConvoReportSubject(
+  convo: ConvoWithDetails,
+  ownDid: string | undefined,
+): ReportSubject | null {
+  if (convo.kind === 'group') {
+    if (!convo.primaryMember) return null
+    return {convoId: convo.view.id, did: convo.primaryMember.did}
+  }
+
+  const lastMessage = convo.view.lastMessage
+  const reportableMessage =
+    ChatBskyConvoDefs.isMessageView(lastMessage) &&
+    lastMessage.sender?.did !== ownDid
+      ? lastMessage
+      : null
+
+  if (reportableMessage) {
+    return {
+      view: 'convo',
+      convoId: convo.view.id,
+      message: reportableMessage,
+    }
+  }
+
+  return {convoId: convo.view.id, did: convo.primaryMember.did}
+}