fix(daemon): make the "keep the node running?" prompt actually appear

Quitting during a witness rebuild is supposed to ask before stopping the
node, because a stop throws the rebuild away and a killed node replays
and redoes it. The guard almost never fired:
- it returned false for any daemon >= v1.3.0, on the false premise that
  v1.3.0 checkpoints a rebuild (it does not; nothing survives a kill)
- it read a 16 KB debug.log tail and compared the last progress line with
  the newest line, so any unrelated line during the rebuild's silent
  second phase turned it off; and on POSIX that file is stale
- daemon_version, which the bypass needed, is zeroed when a busy node
  drops the connection

Now a DaemonBusyTracker (src/daemon/daemon_busy_detect.h, pure) follows
dragonxd's own markers in order: rebuilding (Phase-1 / legacy markers,
ended by the completion line, an abort line or the next UpdateTip) and
replaying (Activating best chain ... Done Loading), reset by a new banner
or "Shutdown: done". It is fed from the owned output on the UI thread,
including the backlog skipped after a restore. The quit guard prompts
when the quit would stop the node and it is rebuilding, or has been
alive but only timing out / refusing as busy for >= 30 s. A replay alone
never prompts: stopping it is safe. The stop loop uses the same signal
to never offer Kill during a rebuild.

Also:
- daemon output offsets are absolute, so a reader that falls behind a
  trim of the 1 MB buffer no longer skips new output (which could hide a
  start or end marker)
- a quit deferred while minimized restores and raises the window, and
  the wizard renders the prompt, so it is never waiting unseen

Verified live with the fake daemon: quitting mid-rebuild now shows the
prompt for a v1.3.0 node and leaves the node alone; a silent busy node
prompts after 30 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/daemon/embedded_daemon.h

@@ -146,16 +146,26 @@ public:
 
     /**
      * @brief Get new output since a given offset (thread-safe).
-     * Returns the new text and updates offset to the current size.
+     * Returns the new text and advances offset. Offsets are absolute positions in everything ever
+     * appended (start from 0), so trimming old output never shifts them: a reader that falls behind a
+     * trim only loses the trimmed part, not output that is still buffered.
      */
     std::string getOutputSince(size_t& offset) const {
         std::lock_guard<std::mutex> lk(output_mutex_);
-        if (offset >= process_output_.size()) {
-            offset = process_output_.size();
+        return outputSince(process_output_, output_total_, offset);
+    }
+
+    /** getOutputSince() on a buffer holding the LAST buffer.size() of `total` bytes ever appended. */
+    static std::string outputSince(const std::string& buffer, size_t total, size_t& offset)
+    {
+        const size_t base = total - buffer.size();   // absolute position of buffer[0]
+        if (offset < base) offset = base;            // trimmed before it was read: resume at what's left
+        if (offset >= total) {
+            offset = total;
             return {};
         }
-        std::string result = process_output_.substr(offset);
-        offset = process_output_.size();
+        std::string result = buffer.substr(offset - base);
+        offset = total;
         return result;
     }
 
@@ -361,6 +371,7 @@ private:
     mutable std::mutex error_mutex_;    // protects last_error_ (written by main + monitor threads)
     mutable std::mutex output_mutex_;  // protects process_output_
     std::string process_output_;
+    size_t output_total_ = 0;          // bytes ever appended (the absolute end of process_output_)
     StateCallback state_callback_;
     
     // Process handle