P4: messages, topics, pubkey

Backend
- api/messages.rs covers send/pop/reply/status with an externally
  tagged MessageDestination enum that matches the daemon's
  {ip|pk: ...} body shape; pop_message uses an inflated request
  timeout to outlast the long-poll window
- api/topics.rs implements default action, topic CRUD, sources
  whitelist, and forward-socket get/set/remove. POST /topics ships
  the raw base64 string as the body (not JSON); path segments are
  percent-encoded inline (topics contain '/' and '+')
- api/pubkey.rs resolves an overlay IPv6 to a hex public key
- poller spawns a third long-poll loop on /messages?peek=false
  that fans every inbound message into a 200-deep ring buffer and
  emits messages://incoming for the UI

Frontend
- messages store: live inbox via the event, persisted outbox via
  tauri-plugin-store keyed under outbox.json
- ComposeMessage form: ip/pk toggle, optional UTF-8 topic and
  payload that get base64-encoded with a TextEncoder-based helper
- MessageList renders printable payloads decoded; binary payloads
  fall back to a "(N bytes binary)" hint
- Topics view: split layout with whitelist on the left, per-topic
  sources/forward editor on the right; default-action toggle is
  surfaced at the top

src-tauri/src/api/mod.rs

@@ -1,6 +1,9 @@
 pub mod admin;
+pub mod messages;
 pub mod peers;
+pub mod pubkey;
 pub mod routes;
+pub mod topics;
 
 use crate::error::{AppError, AppResult};
 use reqwest::{Client, Response};