feat(ui): friendly RPC errors + pause-on-hover/ellipsis toasts

Two of the systemic error-surfacing foundations from the UX audit:

- util::friendlyRpcError() maps the common raw daemon/JSON-RPC errors
  (warming up / -28, locked / -13, method-not-found / -32601, insufficient
  funds, unreachable node) to short translated guidance with a next step,
  falling back to the trimmed raw text. Wired into the send (money) path: the
  toast shows the friendly cause, the overlay keeps the raw text below it for
  diagnostics. The helper is reusable for the remaining raw-error sites.
- Toasts now pause their auto-dismiss while hovered and ellipsize over-long
  text with the full message in a hover tooltip, instead of hard-clipping it
  mid-word (durations were already lengthened earlier).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/notifications.h

@@ -192,6 +192,7 @@ private:
 
     std::deque<Notification> notifications_;
     size_t max_notifications_ = 5;
+    bool toast_hovered_ = false;   // pointer was over the toast last frame → pause its auto-dismiss
     std::chrono::steady_clock::time_point last_drawn_{};
     std::function<void(const std::string&, bool)> console_callback_;