Fix sign conversion warnings in qt_sinks.h

Add static_cast to fix compiler warnings when building with
-Werror=sign-conversion and -Werror=shorten-64-to-32:

1. Cast msg.color_range_start/end to qsizetype when passing to
   QString::fromUtf8(), since QString expects signed qsizetype but
   the message fields are size_t (unsigned).

2. Cast msg.level to size_t when indexing into colors_ array, since
   level_enum is a signed int but array indexing expects size_t.

These casts are safe because:
- qsizetype is guaranteed to be the same size as size_t per Qt docs
- color_range values come from valid string positions
- level values are from a small enum range

Fixes #3321

include/spdlog/sinks/qt_sinks.h

@@ -160,8 +160,8 @@ protected:
             payload = QString::fromUtf8(str.data(), static_cast<int>(str.size()));
             // convert color ranges from byte index to character index.
             if (msg.color_range_start < msg.color_range_end) {
-                color_range_start = QString::fromUtf8(str.data(), msg.color_range_start).size();
-                color_range_end = QString::fromUtf8(str.data(), msg.color_range_end).size();
+                color_range_start = QString::fromUtf8(str.data(), static_cast<qsizetype>(msg.color_range_start)).size();
+                color_range_end = QString::fromUtf8(str.data(), static_cast<qsizetype>(msg.color_range_end)).size();
             }
         } else {
             payload = QString::fromLatin1(str.data(), static_cast<int>(str.size()));
@@ -171,7 +171,7 @@ protected:
                              qt_text_edit_,          // text edit to append to
                              std::move(payload),     // text to append
                              default_color_,         // default color
-                             colors_.at(msg.level),  // color to apply
+                             colors_.at(static_cast<size_t>(msg.level)),  // color to apply
                              color_range_start,      // color range start
                              color_range_end};       // color range end