dm: focus the selected recipient instead of the top conversation

Clicking "Message" on a profile (or selecting anyone you'd never DMed) set the
recipient, but buildDmConversationData discarded it: a recipient with no
existing thread wasn't in conversationMap, so it fell back to conversations[0]
(the top conversation). The inbox silently jumped to the top thread, and any
message sent then went to the wrong person ("says sent but I don't see it").

Fixes:
- setDirectMessageRecipient now focuses the selected recipient's conversation
  so a stale activeDmConversationId can't win.
- buildDmConversationData synthesizes an empty conversation for a
  selected-but-threadless recipient and treats it as active, including an
  activeRemotePubkey fallback so the composer targets the right person.
- The legacy renderProfileMessages list no longer clobbers a non-null
  recipient with threadsToRender[0].

Regression: tests/ui/dm-conversation-focus.test.mjs.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

js/ui/profileModal/ProfileDirectMessageController.js

@@ -242,6 +242,17 @@ export class ProfileDirectMessageController {
     this.updateMessageThreadSelection(nextRecipient);
 
     if (nextRecipient) {
+      // Focus this recipient's conversation so a freshly-selected recipient
+      // (e.g. the "Message" button for someone with no existing thread) isn't
+      // overridden by a stale activeDmConversationId pointing at the top of the
+      // inbox.
+      const actor = this.helper.resolveActiveDmActor();
+      const conversationId = actor
+        ? this.helper.buildDmConversationId(actor, nextRecipient)
+        : "";
+      if (conversationId) {
+        this.setFocusedDmConversation(conversationId);
+      }
       void this.helper.ensureDmRecipientData(nextRecipient);
       this.renderer.setMessagesAnnouncement("Ready to message this recipient.");
     } else if (reason === "clear") {