fix(client): safeBack helper + prevent self-contact-request

1. GO_BACK warning & stuck screens

   When a deep link or direct push put the user on /feed/[id],
   /profile/[address], /compose, or /settings without any prior stack
   entry, tapping the header chevron emitted:
     "ERROR The action 'GO_BACK' was not handled by any navigator"
   and did nothing — user was stuck.

   New helper lib/utils.safeBack(fallback = '/(app)/chats') wraps
   router.canGoBack() — when there's history it pops; otherwise it
   replace-navigates to a sensible fallback (chats list by default,
   '/' for auth screens so we land back at the onboarding).

   Applied to every header chevron and back-from-detail flow:
   app/(app)/chats/[id], app/(app)/compose, app/(app)/feed/[id]
   (header + onDeleted), app/(app)/feed/tag/[tag],
   app/(app)/profile/[address], app/(app)/new-contact (header + OK
   button on request-sent alert), app/(app)/settings,
   app/(auth)/create, app/(auth)/import.

2. Prevent self-contact-request

   new-contact.tsx now compares the resolved address against
   keyFile.pub_key at two points:
     - right after resolveUsername + getIdentity in search() — before
       the profile card even renders, so the user doesn't see the
       "Send request" CTA for themselves.
     - inside sendRequest() as a belt-and-braces guard in case the
       check was somehow bypassed.
   The search path shows an inline error ("That's you. You can't
   send a contact request to yourself."); sendRequest falls back to
   an Alert with the same meaning. Both compare case-insensitively
   against the pubkey hex so mixed-case pastes work.

   Technically the server would still accept a self-request (the
   chain stores it under contact_in:<self>:<self>), but it's a dead-
   end UX-wise — the user can't chat with themselves — so the client
   blocks it preemptively instead of letting users pay the fee for
   nothing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client-app/app/(app)/new-contact.tsx

@@ -18,7 +18,7 @@ import { useSafeAreaInsets } from 'react-native-safe-area-context';
 import { useStore } from '@/lib/store';
 import { getIdentity, buildContactRequestTx, submitTx, resolveUsername, humanizeTxError } from '@/lib/api';
 import { shortAddr } from '@/lib/crypto';
-import { formatAmount } from '@/lib/utils';
+import { formatAmount, safeBack } from '@/lib/utils';
 
 import { Avatar } from '@/components/Avatar';
 import { Header } from '@/components/Header';
@@ -64,9 +64,20 @@ export default function NewContactScreen() {
         if (!addr) { setError(`@${name} is not registered on this chain`); return; }
         address = addr;
       }
+      // Block self-lookup — can't message yourself, and the on-chain
+      // CONTACT_REQUEST tx would go through but serve no purpose.
+      if (keyFile && address.toLowerCase() === keyFile.pub_key.toLowerCase()) {
+        setError("That's you. You can't send a contact request to yourself.");
+        return;
+      }
       const identity = await getIdentity(address);
+      const resolvedAddr = identity?.pub_key ?? address;
+      if (keyFile && resolvedAddr.toLowerCase() === keyFile.pub_key.toLowerCase()) {
+        setError("That's you. You can't send a contact request to yourself.");
+        return;
+      }
       setResolved({
-        address:  identity?.pub_key ?? address,
+        address:  resolvedAddr,
         nickname: identity?.nickname || undefined,
         x25519:   identity?.x25519_pub || undefined,
       });
@@ -79,6 +90,10 @@ export default function NewContactScreen() {
 
   async function sendRequest() {
     if (!resolved || !keyFile) return;
+    if (resolved.address.toLowerCase() === keyFile.pub_key.toLowerCase()) {
+      Alert.alert('Can\'t message yourself', "This is your own address.");
+      return;
+    }
     if (balance < fee + 1000) {
       Alert.alert('Insufficient balance', `Need ${formatAmount(fee + 1000)} (request fee + network).`);
       return;
@@ -96,7 +111,7 @@ export default function NewContactScreen() {
       Alert.alert(
         'Request sent',
         `A contact request has been sent to ${resolved.nickname ? '@' + resolved.nickname : shortAddr(resolved.address)}.`,
-        [{ text: 'OK', onPress: () => router.back() }],
+        [{ text: 'OK', onPress: () => safeBack() }],
       );
     } catch (e: any) {
       setError(humanizeTxError(e));
@@ -114,7 +129,7 @@ export default function NewContactScreen() {
       <Header
         title="Search"
         divider={false}
-        left={<IconButton icon="chevron-back" size={36} onPress={() => router.back()} />}
+        left={<IconButton icon="chevron-back" size={36} onPress={() => safeBack()} />}
       />
       <ScrollView
         contentContainerStyle={{ padding: 14, paddingBottom: 120 }}