Feed pagination, duplicate identity detection, pkarr leak fix, Android SAF

Feed pagination:
- Cursor-based pagination: get_feed_page/get_all_posts_page (20 posts/page)
- Batched engagement queries (3 bulk SQL queries instead of 4 per post)
- IntersectionObserver for infinite scroll (sentinel at midpoint)
- Viewport-based media loading (blobs only load when post enters view)
- Pre-fetch next page immediately after current page renders

Duplicate identity detection:
- Anchor detects when a NodeId is already mesh-connected during initial
  exchange and sets duplicate_active flag in response
- Client skips sync tasks when duplicate detected
- Frontend shows red warning banner

Privacy:
- Fixed pkarr leak: clear_address_lookup() removes default dns.iroh.link
  publishing. Only mDNS (local network) discovery enabled.

Android:
- SAF integration via tauri-plugin-android-fs: exports open native "Save As"
  dialog so users can save to Downloads/Drive/etc.
- Download/export paths use app data dir on Android (writable)
- File picker gated behind desktop cfg (blocking_pick not on Android)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/core/src/node.rs

@@ -1,6 +1,6 @@
 use std::net::SocketAddr;
 use std::path::{Path, PathBuf};
-use std::sync::atomic::{AtomicU64, Ordering as AtomicOrdering};
+use std::sync::atomic::{AtomicBool, AtomicU64, Ordering as AtomicOrdering};
 use std::sync::Arc;
 
 use tracing::{debug, info, warn};
@@ -30,6 +30,8 @@ pub struct Node {
     pub blob_store: Arc<BlobStore>,
     secret_seed: [u8; 32],
     bootstrap_anchors: tokio::sync::Mutex<Vec<(NodeId, iroh::EndpointAddr)>>,
+    /// True if an anchor reported another instance of this identity is already active
+    pub duplicate_detected: Arc<AtomicBool>,
     #[allow(dead_code)]
     profile: DeviceProfile,
     pub activity_log: Arc<std::sync::Mutex<ActivityLog>>,
@@ -136,6 +138,7 @@ impl Node {
             blob_store,
             secret_seed,
             bootstrap_anchors: tokio::sync::Mutex::new(Vec::new()),
+            duplicate_detected: Arc::new(AtomicBool::new(false)),
             profile,
             activity_log: activity_log_ref,
             last_rebalance_ms,
@@ -927,6 +930,34 @@ impl Node {
         Ok(self.decrypt_posts(raw, &group_seeds))
     }
 
+    pub async fn get_feed_page(
+        &self,
+        before_ms: Option<u64>,
+        limit: usize,
+    ) -> anyhow::Result<Vec<(PostId, Post, PostVisibility, Option<String>)>> {
+        let (raw, group_seeds) = {
+            let storage = self.storage.get().await;
+            let posts = storage.get_feed_page(before_ms, limit)?;
+            let seeds = storage.get_all_group_seeds_map().unwrap_or_default();
+            (posts, seeds)
+        };
+        Ok(self.decrypt_posts(raw, &group_seeds))
+    }
+
+    pub async fn get_all_posts_page(
+        &self,
+        before_ms: Option<u64>,
+        limit: usize,
+    ) -> anyhow::Result<Vec<(PostId, Post, PostVisibility, Option<String>)>> {
+        let (raw, group_seeds) = {
+            let storage = self.storage.get().await;
+            let posts = storage.list_posts_page(before_ms, limit)?;
+            let seeds = storage.get_all_group_seeds_map().unwrap_or_default();
+            (posts, seeds)
+        };
+        Ok(self.decrypt_posts(raw, &group_seeds))
+    }
+
     fn decrypt_posts(
         &self,
         posts: Vec<(PostId, Post, PostVisibility)>,