fix(client): profile polish + proper back stack + dev feed seed

Profile screen
  - Missing safe-area top inset on the header — fixed by wrapping the
    outer View in paddingTop: insets.top (matches the rest of the tab
    screens).
  - "Чат" button icon + text sat on two lines because the button used
    column layout by default. Rewritten as a full-width CTA pill with
    flexDirection: row and alignItems: center → chat-bubble icon and
    label sit on one line.
  - Dedicated "Копировать адрес" button removed. The address row in
    the info card is now the tap target: pressing it copies to clipboard
    and flips the row to "Скопировано" with a green check for 1.8s.
  - Posts tab removed entirely. User's right — a "chat profile" has no
    posts concept, just participant count + date + encryption. The
    profile screen is now a single-view info card (address, encryption
    status, added date, participants). Posts are discoverable via the
    Feed tab; a dedicated /feed/author/{pub} screen is on the roadmap
    for browsing a specific user's timeline.

Back-stack navigation
  - app/(app)/profile/_layout.tsx + app/(app)/feed/_layout.tsx added,
    each with a native <Stack>. The outer AnimatedSlot is stack-less
    (intentional — it animates tab switches), so without these nested
    stacks `router.back()` from a profile screen had no history to pop
    and fell through to root. Now tapping an author in the feed → back
    returns to feed; opening a profile from chat header → back returns
    to the chat.

Dev feed seed
  - lib/devSeedFeed.ts: 12 synthetic posts (text-only, mix of hashtags,
    one with has_attachment, varying likes/views, timestamps from "1m
    ago" to "36h ago"). Exposed via getDevSeedFeed() — stripped from
    production via __DEV__ gate.
  - Feed screen falls back to the dev seed only when the real API
    returned zero posts. Gives something to scroll / tap / like-toggle
    before the backend has real content; real data takes over as soon
    as it arrives.

Note: tapping a mock post into detail will 404 against the real node
(the post_ids don't exist on-chain). Intentional — the seed is for
scroll + interaction feel, not deep linking.

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

client-app/app/(app)/feed.tsx

@@ -27,6 +27,7 @@ import {
   fetchTimeline, fetchForYou, fetchTrending, fetchStats, bumpView,
   type FeedPostItem,
 } from '@/lib/feed';
+import { getDevSeedFeed } from '@/lib/devSeedFeed';
 
 type TabKey = 'following' | 'foryou' | 'trending';
 
@@ -71,6 +72,13 @@ export default function FeedScreen() {
           break;
       }
       if (seq !== requestRef.current) return;   // stale response
+
+      // Dev-only fallback: if the node has no real posts yet, surface
+      // synthetic ones so we can scroll + tap. Stripped from production.
+      if (items.length === 0) {
+        items = getDevSeedFeed();
+      }
+
       setPosts(items);
 
       // Batch-fetch liked_by_me (bounded concurrency — 6 at a time).