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/protocol.rs

@@ -184,6 +184,9 @@ pub struct InitialExchangePayload {
     /// CDN cache pressure: 0-255 availability score (255 = lots of capacity)
     #[serde(default)]
     pub cache_pressure: Option<u8>,
+    /// Set by anchor when it detects this NodeId is already connected from a different address
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub duplicate_active: Option<bool>,
 }
 
 /// Incremental N1/N2 changes