feat(chat): the preview's button knows whether it can be pressed

Phase 4 of docs/npub-profile-preview.md. Phases 1-3 moved the twenty-second
dead end one screen later: a person found on an indexer, a press, and
InitiatingNewChat waiting for a key package that someone who has never opened
Curare will never have published. The preview now reads the answer before the
press. A room that already exists opens; a key package that is here enables
"Start new chat"; one that is not, once the relays have had their time,
disables it with the reason under it and a try-again that asks the DM relays
alone.

The existing-room check was a loop with a TODO inside initiateNewChat; it is
ChatRepository.findDirectMessageRoom now -- a room whose members are exactly
the two keys, or the one key when they are the same -- read by both screens and
pinned by a test on an in-memory database. The old size-one branch would have
opened any room the peer sat in alone, whoever they were.

Both halves are observed together, as the chat screen does: a key package
arriving after the profile touches a different table, and a collect on the
profile alone would never see it. A room is read once up front, because a key
package is meant to be used once and the relays need not hold a fresh one for
someone you already talk to.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Pulled-From: curated/curated@d649098504

composeApp/src/commonMain/composeResources/values/strings.xml

@@ -488,4 +488,7 @@
     <string name="find_profile">Find profile</string>
     <string name="looking_for_this_profile_on_the_relays">Looking for this profile on the relays.</string>
     <string name="start_new_chat">Start new chat</string>
+    <string name="you_already_have_a_chat_with_s">You already have a chat with %1$s.</string>
+    <string name="checking_whether_s_can_receive_messages_here">Checking whether %1$s can receive messages here.</string>
+    <string name="s_hasn_t_set_up_messaging_on_mantra_yet">%1$s hasn't set up messaging on Mantra yet. Try again later, or ask them to open Mantra.</string>
 </resources>