refactor: extract utf8 truncation logic to utility

Move the UTF-8 visual-width measurement and truncation helper functions
from the mpris module to a common utility
(include/util/utf8_string.hpp and src/util/utf8_string.cpp).

This decouples string truncation and width measurement
from the mpris module, allowing other modules (like mpd) to reuse.
The helper function `utf8_truncate` (formerly `truncate` in mpris)
and `utf8_width` is exported under the `waybar::util` namespace,
while the low-level `measure_and_truncate` (formerly `utf8_truncate`)
is encapsulated in an anonymous namespace in src/util/utf8_string.cpp
to avoid unnecessary API exposure.

No functional changes were made to the mpris module's behavior.

src/util/utf8_string.cpp

@@ -0,0 +1,62 @@
+#include <glib.h>
+
+#include <string>
+#include <util/utf8_string.hpp>
+
+namespace waybar::util {
+
+namespace {
+// Wide characters count as two, zero-width characters count as zero
+// Modifies str in-place (unless width = std::string::npos)
+// Returns the total width of the string pre-truncating
+size_t measure_and_truncate(std::string& str, size_t width = std::string::npos) {
+  if (str.length() == 0) return 0;
+
+  const gchar* trunc_end = nullptr;
+
+  size_t total_width = 0;
+
+  for (gchar *data = str.data(), *end = data + str.size(); data != nullptr;) {
+    gunichar c = g_utf8_get_char_validated(data, end - data);
+    if (c == -1U || c == -2U) {
+      // invalid unicode, treat string as ascii
+      if (width != std::string::npos && str.length() > width) str.resize(width);
+      return str.length();
+    } else if (g_unichar_iswide(c)) {
+      total_width += 2;
+    } else if (!g_unichar_iszerowidth(c) && c != 0xAD) {  // neither zero-width nor soft hyphen
+      total_width += 1;
+    }
+
+    data = g_utf8_find_next_char(data, end);
+    if (width != std::string::npos && total_width <= width && !g_unichar_isspace(c))
+      trunc_end = data;
+  }
+
+  if (trunc_end) str.resize(trunc_end - str.data());
+
+  return total_width;
+}
+}  // namespace
+
+size_t utf8_width(const std::string& str) {
+  return measure_and_truncate(const_cast<std::string&>(str));
+}
+
+void utf8_truncate(std::string& s, const std::string& ellipsis, size_t max_len) {
+  if (max_len == 0) {
+    s.resize(0);
+    return;
+  }
+  size_t len = measure_and_truncate(s, max_len);
+  if (len > max_len) {
+    size_t ellipsis_len = utf8_width(ellipsis);
+    if (max_len >= ellipsis_len) {
+      if (ellipsis_len) measure_and_truncate(s, max_len - ellipsis_len);
+      s += ellipsis;
+    } else {
+      s.resize(0);
+    }
+  }
+}
+}  // namespace waybar::util