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/lib/utils.ts

@@ -21,3 +21,37 @@ export function shortenIpv6(addr: string): string {
   if (addr.length <= 24) return addr;
   return `${addr.slice(0, 10)}…${addr.slice(-8)}`;
 }
+
+// ─── base64 helpers (UTF-8 safe) ─────────────────────────────────────────────
+
+export function utf8ToBase64(s: string): string {
+  const bytes = new TextEncoder().encode(s);
+  let bin = "";
+  for (const b of bytes) bin += String.fromCharCode(b);
+  return btoa(bin);
+}
+
+export function base64ToUtf8(b64: string): string {
+  try {
+    const bin = atob(b64);
+    const bytes = new Uint8Array(bin.length);
+    for (let i = 0; i < bin.length; i++) bytes[i] = bin.charCodeAt(i);
+    return new TextDecoder("utf-8", { fatal: false }).decode(bytes);
+  } catch {
+    return b64;
+  }
+}
+
+export function isPrintableUtf8(b64: string): boolean {
+  try {
+    const bin = atob(b64);
+    for (let i = 0; i < bin.length; i++) {
+      const c = bin.charCodeAt(i);
+      // Reject NUL and most C0 except CR/LF/TAB.
+      if (c === 0 || (c < 0x20 && c !== 9 && c !== 10 && c !== 13)) return false;
+    }
+    return true;
+  } catch {
+    return false;
+  }
+}