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/ProfileDirectMessageRenderer.js

@@ -566,11 +566,14 @@ export class ProfileDirectMessageRenderer {
       }
 
       const activeRecipient = this.controller.helper.resolveActiveDmRecipient();
-      const hasActiveRecipient =
-        activeRecipient &&
-        threadsToRender.some((thread) => thread.remoteHex === activeRecipient);
 
-      if (threadsToRender.length && !hasActiveRecipient) {
+      if (activeRecipient) {
+        // Respect an explicitly-selected recipient even if it has no thread yet
+        // (e.g. the "Message" button for someone you've never DMed). Defaulting
+        // to threadsToRender[0] here is what made the inbox jump to the top
+        // conversation instead of the person you chose.
+        this.updateMessageThreadSelection(activeRecipient);
+      } else if (threadsToRender.length) {
         this.controller.setDirectMessageRecipient(threadsToRender[0].remoteHex, {
           reason: "thread-default",
         });
@@ -579,8 +582,6 @@ export class ProfileDirectMessageRenderer {
           threadsToRender[0].remoteHex,
         );
         this.controller.setFocusedDmConversation(conversationId);
-      } else if (hasActiveRecipient) {
-        this.updateMessageThreadSelection(activeRecipient);
       }
 
       this.profileMessagesList.classList.remove("hidden");