fix(feed): visible post divider + reliable FAB positioning

- Post divider was on each PostCard's outer Pressable as borderBottom
  (#222), which was barely visible on OLED black and disappeared
  entirely in pressed state (the pressed bg ate the line). Moved the
  seam to a dedicated PostSeparator component (1px, #2a2a2a) wired as
  FlatList's ItemSeparatorComponent on both /feed (timeline / for-you
  / trending) and /feed/tag/[tag]. Also bumped inter-card vertical
  padding (14-16 top / 16-20 bottom) so cards have real breathing room
  even before the divider.
- FAB position was flaky: with <Stack> at the (app) level the overlay
  could end up positioned against the Stack's card view instead of the
  tab container, which made the button drift around and stick against
  unexpected edges. Wrapped it in an absoluteFill container with
  pointerEvents="box-none" — the wrapper owns positioning against the
  tab screen, the button inside just declares right: 14 / bottom: N.
  Bumped bottom offset to `max(insets.bottom, 8) + 70` so the FAB
  always clears the 5-icon NavBar with ~14px visual gap on every
  device. Shadow switched from blue-cast to standard dark for better
  depth perception on dark backgrounds.

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

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

@@ -21,7 +21,7 @@ import { useSafeAreaInsets } from 'react-native-safe-area-context';
 import { router } from 'expo-router';
 
 import { TabHeader } from '@/components/TabHeader';
-import { PostCard } from '@/components/feed/PostCard';
+import { PostCard, PostSeparator } from '@/components/feed/PostCard';
 import { useStore } from '@/lib/store';
 import {
   fetchTimeline, fetchForYou, fetchTrending, fetchStats, bumpView,
@@ -218,6 +218,7 @@ export default function FeedScreen() {
             onDeleted={onDeleted}
           />
         )}
+        ItemSeparatorComponent={PostSeparator}
         refreshControl={
           <RefreshControl
             refreshing={refreshing}
@@ -250,29 +251,43 @@ export default function FeedScreen() {
         contentContainerStyle={posts.length === 0 ? { flexGrow: 1 } : undefined}
       />
 
-      {/* Floating compose button — pinned to the bottom-right corner
-          with 14px side inset. Vertical offset clears the 5-icon NavBar
-          (which lives below this view in the same layer) by sitting
-          ~14px above its top edge. */}
-      <Pressable
-        onPress={() => router.push('/(app)/compose' as never)}
-        style={({ pressed }) => ({
+      {/* Floating compose button.
+       *
+       * Wrapped in a StyleSheet.absoluteFill container with pointerEvents
+       * "box-none" so only the FAB captures touches — taps anywhere else
+       * pass through to the FlatList below.
+       *
+       * Inside the wrapper, alignSelf: 'flex-end' pins to the right;
+       * bottom inset leaves ~14px clearance above the NavBar (≈56px tall
+       * + safe-area-bottom). Explicit `right: 14` is belt-and-braces
+       * for RTL / platform quirks where alignSelf alone might not pin. */}
+      <View
+        pointerEvents="box-none"
+        style={{
           position: 'absolute',
-          right: 14,
-          bottom: Math.max(insets.bottom, 0) + 62,
-          width: 56, height: 56,
-          borderRadius: 28,
-          backgroundColor: pressed ? '#1a8cd8' : '#1d9bf0',
-          alignItems: 'center', justifyContent: 'center',
-          shadowColor: '#1d9bf0',
-          shadowOffset: { width: 0, height: 4 },
-          shadowOpacity: 0.4,
-          shadowRadius: 8,
-          elevation: 6,
-        })}
+          left: 0, right: 0, top: 0, bottom: 0,
+        }}
       >
-        <Ionicons name="create-outline" size={24} color="#ffffff" />
-      </Pressable>
+        <Pressable
+          onPress={() => router.push('/(app)/compose' as never)}
+          style={({ pressed }) => ({
+            position: 'absolute',
+            right: 14,
+            bottom: Math.max(insets.bottom, 8) + 70,
+            width: 56, height: 56,
+            borderRadius: 28,
+            backgroundColor: pressed ? '#1a8cd8' : '#1d9bf0',
+            alignItems: 'center', justifyContent: 'center',
+            shadowColor: '#000',
+            shadowOffset: { width: 0, height: 4 },
+            shadowOpacity: 0.5,
+            shadowRadius: 6,
+            elevation: 8,
+          })}
+        >
+          <Ionicons name="create-outline" size={24} color="#ffffff" />
+        </Pressable>
+      </View>
     </View>
   );
 }

client-app/app/(app)/feed/tag/[tag].tsx

@@ -14,7 +14,7 @@ import { useSafeAreaInsets } from 'react-native-safe-area-context';
 
 import { Header } from '@/components/Header';
 import { IconButton } from '@/components/IconButton';
-import { PostCard } from '@/components/feed/PostCard';
+import { PostCard, PostSeparator } from '@/components/feed/PostCard';
 import { useStore } from '@/lib/store';
 import { fetchHashtag, fetchStats, type FeedPostItem } from '@/lib/feed';
 
@@ -94,6 +94,7 @@ export default function HashtagScreen() {
             onStatsChanged={onStatsChanged}
           />
         )}
+        ItemSeparatorComponent={PostSeparator}
         refreshControl={
           <RefreshControl
             refreshing={refreshing}