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/lib/utils.ts

@@ -1,5 +1,23 @@
 import { clsx, type ClassValue } from 'clsx';
 import { twMerge } from 'tailwind-merge';
+import { router } from 'expo-router';
+
+/**
+ * Navigate back, or fall back to a sensible default route if there's
+ * no screen to pop to.
+ *
+ * Without this, an opened route entered via deep link / direct push
+ * (profile, feed/[id], etc.) would emit the "action 'GO_BACK' was not
+ * handled by any navigator" dev warning and do nothing — user ends up
+ * stuck. Default fallback is the chats list (root of the app).
+ */
+export function safeBack(fallback: string = '/(app)/chats'): void {
+  if (router.canGoBack()) {
+    router.back();
+  } else {
+    router.replace(fallback as never);
+  }
+}
 
 export function cn(...inputs: ClassValue[]) {
   return twMerge(clsx(inputs));