feat(chat): Accept/Block card for unsolicited contact requests

An unsolicited first message from a stranger (an incoming ContactRequest, with
nothing sent by us and the peer not in contacts) now shows an Accept / Block
card above the thread instead of looking like a normal conversation. Accept adds
the peer to the address book (which clears the request state); Block deletes and
blocks the conversation (reversible from the Blocked manager). ConvSummary gains
a derived pendingRequest flag. (The list-row "Request" pill and a header Block
button were left out to avoid the fragile hand-drawn header-toolbar geometry.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/i18n.cpp

@@ -232,6 +232,11 @@ void I18n::loadBuiltinEnglish()
         strings_["chat_empty_hint"] = "No conversations yet. Messages you receive will appear here.";
         strings_["chat_you"] = "You";
         strings_["chat_contact_request"] = "contact request";
+        strings_["chat_request_title"] = "New contact request";
+        strings_["chat_request_body"] = "This person isn't in your contacts. Accept to add them, or block to delete this conversation and ignore them.";
+        strings_["chat_request_accept"] = "Accept";
+        strings_["chat_request_block"] = "Block";
+        strings_["chat_request_accepted"] = "Contact added.";
         strings_["chat_send_failed"] = "not sent";
         strings_["chat_status_sending"] = "Sending… waiting for your wallet to broadcast it";
         strings_["chat_status_sent"] = "Sent to the network — it arrives after it confirms (about a minute)";